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.

CM change id: I754d7fdcaf27c23dfccf9d56291956e70b23f63e

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My tester says for people with bad vision it is easier to set up more
comfortable font size with this scroller, than with usual AOSP selector,
that's why I've taken liberty to fetch this.

Conflicts:
	res/values/attrs.xml
	res/values/cm_strings.xml
	src/com/android/settings/DisplaySettings.java

Change-Id: Ie2fe07214973e9be781f07199066aff1eb55534a