Assorted layout improvements to the Color Selector Settings pane

Hi all,

In this MR I apply a bit of the existing design language to the Color Selector preference panel. This was spurred by word-of-mouth complaints about readability and lack of flow when switching between the panels on Windows. And on Linux, while testing this, I found that on a native build (here Manjaro with Gnome) the selector would actually collapse on itself. I put an example as the first screenshot.

To fix that, and also improve on the accessibility, I simplified and ported the panel to the form layout, ensured that elements take their native sizes, and enforced layout through size policies instead of spacers or minimum sizes. (The latter was a key contribution to the collapse behaviour exhibited under the Fusion theme.)

I've kept these changes in different commits to ease review and revert if those are needed.

Screenshots

Before	After

Test Plan

Build Krita, open the Preferences > Color Selector Settings pane, and check that all is rendered well in your system.

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.