Global Theme KCM Redesign

This merge request redesigns Global Theme, and makes reuse of some existing seemingly unused code in the KCM. The redesign is largely based off @ngraham's design ideas, as well as some of my own, subject to change as per VDG feedback.

Highlights of this redesign include

• Kirigami sheet for choosing what is applied
• The 'roles' code is now made use of for the above
• New roles, and fixes to existing roles
• Desktop Layout can now be applied separately without applying the whole Global Theme
• There are more options for controlling applying in Global Themes now, including Fonts, Splash Screen, and Lock Screen

Screenshots

Before (borrowing this image as it shows Global Theme in its current state):

After:

Important Notes

• Switching between More and Less Options will reset the checkbox states - this is intentional as not resetting the checkboxes during the switch between these views will allow the Global Theme checkbox to become inconsistent with the More Options checkboxes
• Desktop Layout's checkbox is cloned into More Options for the above reason
• I made a More Options section because the abundance of checkboxes being on the main dialog instead might be confusing for users
• You'd want to also merge !1535 (merged) to give Breeze a layouts file in its Global Theme, as it currently does not have one which therefore makes it impossible to apply the Breeze Desktop Layout since to the KCM it doesn't have a Desktop Layout whatsoever