gs-category-tile: Update to match new design

Here are some changes to make further progress on making the category tiles match the new sidebarless design.

The changes make the category tiles bigger and more padded, and implement per-category styling for them. The categories are moved up the overview page (to just below the carousel), but there’s currently no support for bigger category tiles (Play and Develop in the designs); that will follow in another MR once this one’s reviewed.

Open questions/issues to note:

• The category icons don’t match the designs because I couldn’t find the icons that are in the designs; please point me towards better icons if what’s implemented needs changing
• I’ve done a reasonable approximation of the gradients, colours, etc. from the designs, but without knowing exactly the colour stops and angles used in the design, I can’t make it exact, so CSS tweaking will probably be required — please suggest concrete changes, ideally as GitLab suggestions on the diff in this MR
• I’m unsure what styling should be applied to the category tiles in HighContrast mode, since they’re visually quite different from normal buttons in the normal Adwaita theme (so currently styling in HighContrast is broken)
• The background breaks on the Work tile when hovering over it and I haven’t investigated why (because the style is likely to be tweaked)

Screenshot:

/cc @aday, @bertob for design review

Helps: #1111 (closed)