[kcms/fonts] Add preview and move AA stuff to a separate page
Commit number 1 commit adds a preview area to the Fonts KCM to help users visualize changes they've made, including how changing the font size resizes UI elements.
Commit number 2 moves the anti-aliasing stuff to a sub-page.
Commit number 3 adds an inset shadow to the preview area.
Here's how everything looks now: