Crash occurs in diff editor when there is a block selection
and you are changing context lines number.

This workarounds the crash, but it seems that
when BaseTextEditorWidget has a blockSelection
calling "clear()" and "setPlainText()"
causes the crash. When I turn off the
blockSelection before calling "clear()" it helps.

Change-Id: I0133862a2d7e2914c16368b7efa9986b4d56ff39
Reviewed-by: David Schulz <david.schulz@digia.com>