Cursor movement makeover

Reimplement the left/right cursor movement with an algorithm which is indepedent of text rendering.
This should be cleaner and less buggy. The new implementation should also be more efficient.

Note that from a user's view point this commit should behave very similar to the previous implementation,
with only a few fixes for exterme cases when combining RTL and LTR texts.

For example this commit should fix the case of "Re: <RTL>", where previously the cursor could
not be returned to the start of the string with the arrows.

Change-Id: I75a65fbd2fad8d5d944445d0a2428e84a3b82d7c