DB Explorer: implement missing features

Hi!

This MR proposes implementing the two missing features in Halla's database explorer: per-resource versions listing and per-tag resource listings. To achieve that, I had to do some fixes to the resource and tag models, as well as implement missing data marshalling.

I've also taken the opportunity to improve on the layout and make it more friendly to the user, mainly by rearranging some tables, adding group boxes to label bits of the Resources tab, and fixing the rendering of controls in the Resources and Tags tables.

Before	After

Questions

Should we shrink the thumbnails provided for DecorationRole? These make row resizing impossible because they balloon the height. cc @szaman since you were working on caching these.

Test Plan

Build Krita and call up the Resource Cache Database Explorer.

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.

/cc @woltherav