refactoring visual cursor movement functions

refactoring visual cursor movement functions

Specifically, moved the functions which actually move left or right from Text
into Cursor, and changed the semantics so that they now return whether or not
the cursor has moved. The functions in Text now use the new functions in Cursor.

This change is being made in preparation of the Word-level visual movement.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@24601 a592a061-630c-0410-9148-cb99ea01b6c8