Improve the look of Welcome Page with a few small changes

I'm not an expert graphic designer but in my personal opinion, the current Welcome Page widget has a few graphic design issues, and since Krita is an artsy program, we should strive to make it nice looking, otherwise it might give the wrong impression to professional artists. Even if this Welcome Page is "only temporary", we already shipped a few versions of Krita with it, so it must be up to standards even if we plan something better for it in the future.

I think the previous Welcome Page had good graphic design, but of course the new one is functionally better due to the Recent Files thumbnails and the banner, so we gotta make this one good looking too.

Things I consider issues with the current Welcome Page:

• the banner starts and ends in places misaligned with the borders between the sections below
• the middle and right sections have outlines but the left section doesn't
• the New File and Open File icons + action text + shortcut on the side look a bit weird
• the items inside the Start and Community sections are misaligned
• The News section is (by default) a huge empty void with a very small text in one line going exactly from one edge to the other
• The Recent Files, when they have all different dimensions, look incoherent - the faint outline helps a bit but not enough imho
• the "Krita is an open source program..." footnote is so low that no one will ever read it (it was much higher earlier)

Comparison of the old design and the current design, showcasing the issues:

Things I did:

• I mocked up a longer banner
  • note that it is fading on the sides, otherwise it's a bit too much contrast there - we'd need an artist, preferably the one who made the original banner (Raghukamath?), to make a proper one with those qualities (the banner in the MR is also a bit thinner since the "automatic scaling" option kicked in - it wasn't intentional)
  • the longer banner solves the issue of the page not being divided into perfect thirds, since it ends at the ends of the whole page, not around the borders between sections
  • the banner is pretty and easily gets attention, I love it, let's make it fit the rest of the page more, maybe less people would close it? (that's just assumption though. I personally don't want to close it already)
• I added a faint background behind the recent files, so they all look more squarish and more coherent (imho)
• I changed the New File and Open File back to the original icon-action-shortcut horizontal layout - if someone disagrees (possibly the person who changed it first?), please let me know, would be good to discuss

Comparison between the current design and the current (at the time of writing, January 12th, 2024) state of this MR:

Things to do:

• !!! change the banner into a well-painted one
• !!! make sure the X on the banner is visible and easy to access and click
• remove the toolbar
• add outline to the left section or sections (can be one big or two smaller ones)
• make the News widget look better when empty (add info that enabling news enables notifications about new versions of Krita as well)
• make the "missing thumbnail" thumbnails in Recent Files also have this background or at least an outline, otherwise they break the look

Questions:

• Do we need the shortcuts on the side of New File and Open File?
• Do we need that toolbar on top? It only repeats the actions that are already shown below
• What to do with the footnote?

Test Plan

Look at the pictures, look at your Krita, and tell me what you think. Criticism & ideas definitely welcome.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.