WIP: [kcms/icons] Turn icon size popup into it's own page

Rather than selecting a a specific item to change and having a disconnected control that alters the selected item, we can use the space and present all the icon sizes each with their own control connected with the label. Follow up to

We can also reduce the code needed significantly and everything still works, including key handling.

Follow up to plasma-workspace!170 (merged)

WIP as I do UX, but I don't do UI. Someone probably wants to redo the delegate, especially the margins and such. Feel free to just push on top of this branch.

(also why is that patch against p-w and mine against p-d?)