masks: first steps for restricted masks edit mode
masks: implement restricted edit mode

you enter restricted edit mode by CONTROL-click on the "show masks and edit"
icon. in restricted edit mode shapes are protected against accidental deleting,
shifting and enlarging/shrinking. you can still act on single nodes and on the
feathering border. main purpose: eases fine-tuning of shapes (esp. path shapes).
Merge branch 'mymasks'