QmlDesigner.PropertyEditor: Adding dirty flag to LineEdit

This patch ensures that we only commit data to the backend
if the text actually changed. The signal onEditingFinished is triggered
even if the user did not edit the string.

Change-Id: I2d54f3d79a1c60525e4c9744b364cc014bd407ce
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>