Check if widget inherits from QTextEdit when drawing the frame

There can be cases where the accessibility plugin is not available so
rather than have an incorrect style in those cases we check if the
widget inherits QTextEdit.

Change-Id: Ia514ce61f24ef016f56c6dce103f90f699b4048a
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>