Style Switcher between Fusion, Breeze, or other Qt styles

Add a way in the GUI to set a Qt style. This checks the system and gives all the options that are available. If a person manually sets a style, that decision is saved off to kritarc. If it was never set, the system selected style is used. You can access/change the style from the main menu.

Test Plan

I checked to make sure it worked when there was nothing in kritarc

I checked to make sure changing the value saved it to kritarc

I checked to make sure loading krita would read the kritarc and load the style set

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.