[kritamultiarch] Assorted enhancements and warning fixes from the upcoming refactor

This MR contains the least controversial fixes I made as part of the preparations for the kritamultiarch refactor.

Nothing too concerning except for the KoColorSpace equality change-- I noticed that we used a full server plus a QHash to wrap around something as simple as qHash(name of the color space). I removed that in favour of directly using qHash.

Test Plan

Build Krita and run its tests.

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.