Respect _applyMatrix setting when calling #applyMatrix() in Group#_changed.

Required for Clip to work correctly.
Fix issues with SVG Import and matrices on groups.
Pass right value for dontNotify.
Rename #_applyMatrix to #_transformContent.

And swap names with private function...
Define Group#transformContent as a boolean to control #applyMatrix() behavior.

And use it for SVG Import.
Remove Clip class again.

Group can handle that.