Display Settings: Better font size control

Use a SeekBar instead of a list/spinner to determine the font scaling.
For consistency, an approximate size (small, normal, etc.)
 and the exact percentage are displayed.
Show a preview of the selected font size immediately

Ported from CM 10.2, squashed commits 46678 and 49167.
Credit to Veeti Paaananen for the latter commit.

Change-Id: I754d7fdcaf27c23dfccf9d56291956e70b23f63e