Adds two actions to indent and unindent a selection.

Add actions to the basetexteditor to indent and unindent a selection.  These actions
are equivalent to Key_Tab and Key_BackTab respectively.  The advantage of having
actions here is the user can override them and assign any key binding they want.
Thus, I could assign Ctrl+I and Shift+Ctrl+I to them and have the same behavior
as Kate.

Change-Id: I0a305a7b45018072a78f2880ea15650ea92095a1
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>