Implement majority of the settings re-organization

Part of #15.

This commit re-organizes categories to match what was agreed to in the above-mentioned Issue.

It does not fully implement everything there because we're still missing a few technical pieces that are blockers for fully completing the project:

• No infrastructure for embedding KCMs in other KCMs. So in places where we envisioned embedding, a status-quo-style child category is used instead. Not quite as nice, but close enough for now.
• No ability to hide KCMs (or categories) based on hardware availability, so more KCMs appear in the "Input & Output" category than were originally envisioned.
• The Date & Time KCM doesn't build against Qt6 yet, so it doesn't appear in the "Regional Settings" category as expcted.
• No "mouse and touchpad" icon, so the Mouse and Touchpad KCMs are still ungrouped.
• Kept the "Workspace Behavior" category name for now instead of renaming it to "Search & Shortcuts" as envisiones, because the other KCMs besides Search and Shortcuts that currently live in that category cannot yet be removed from it in the absence of the embedding feature or a better logical home for them.

Still, this MR plus the invididual ones in other repos (listed in the "Related Merge Requests" section of #15) amount to most of the project from a UX perspective, and the remaining pieces can trickle in over time and preserve the overall structure implemented here.

Please avoid questioning the entire project or proposing radical overhauls of the organization, as this is an implementation of what was agreed to in #15.

Here's what System Settings' sidebar looks like now--or would, if you had a screen tall enough to make it not scrollable. 😄