Bugs 412232 428495: Better layout and themes support for Welcome screen

Links:

• https://krita-artists.org/t/layout-of-the-kiswelcomepagewidget/13808
• https://bugs.kde.org/show_bug.cgi?id=412232
• https://bugs.kde.org/show_bug.cgi?id=428495

What's changed:

• Better layout of welcome screen: uniformed margins, spacing, links heights.
• Icons for recent documents made square and the same size.
• Moved link labels parameters to .ui
• Fixed theme support for NewsWidget. Layout of the News
• Removed background from NewsWidget to make it look consistent with other Welcome widgets.
• As suggested limited news to 5 items and added a red heart to "Support Krita" and moved it down.
• Fixed a bug with multiple presses on recent docs.
• Fixed removing the existing history entry alongside with not existing.
• Changed mouse pointer to all underlined items to pointing hand

I didn't fixed the theme itself, so the barely visible icons mentioned in 412232 aren't fixed.

Screenshots

Screenshots:

BEFORE

AFTER

Neutral Theme AFTER

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.