Introduce hook for text cursor changes in QTextEdit and QPlainTextEdit

Introduce hook for text cursor changes in QTextEdit and QPlainTextEdit

Currently it is hard for Q{Plain}TextEdit subclasses to keep track
of changes of the text cursor by direct calls to setTextCursor from
arbitrary code. By redirecting the setTextCursor to a virtual
function subclasses get the chance to intercept such calls.

Change-Id: Id92d9c86928b97285b0f5b7b39790442dfac3e50
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>