[Fonts KCM] remove filler words from anti-aliasing settings' labels

Summary:
This is a new version of D13593 with only the non-controversial changes (plus one new change, because I just can't resist stirring the pot :) ):

• "Use Anti-aliasing" -> "Anti-aliasing": The verb is not needed and in fact becomes awkward with the combobox's options
• "Sub-pixel rendering type" -> "Sub-pixel rendering"; "type" is just useless filler that lengthens the string and doesn't add any information
• "Hinting style" -> "Hinting" -> same thing; "style" is just filler

Also added context to help translators come up with the best translations for the now shorter strings.

Test Plan:

Reviewers: VDG, Plasma, mart

Reviewed By: VDG, Plasma, mart

Subscribers: abetts, svenmauch, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15738