kcms/tablet: Overhaul tablet UI

I posted a few concept screenshots of what I wanted, but here it is in actual-code-form:

(NOTE: Screenshots are a bit outdated, do not use them. The header color has changed.)

The code is definitely not ready to ship, don't worry about how that looks. I'm curious if this looks good and the flow makes some sort of sense. The goal is to make space for more options and reduce how long the page is going to get once we have stuff like pen pressure options, maybe a relative mode and better pad button layouts. ATM it's going to look barren, but I think that's better than having to scroll so much.