masks and blending: revamped unified GUI
Merge branch 'master' into blendmasks
masks and blending: add mask combination options: substractive/additive, normal/inversed
Merge branch 'master' into blendmasks
masks and blending: some wording changes
masks and blending: some ui fixes
Merge branch 'master' into blendmasks
masks and blending: fix some logical flaws
Merge branch 'master' into blendmasks
masks and blending: minor gui changes
masks and blending: add functionality for deprecated blend modes

* make blend modes 'inverse' deprecated as masks can now be inverted
* introduce new blend mode 'difference' (better working in Lab) and make
  old one deprecated
masks and blending: a few wording changes in UI
masks and blending: fix bug in legacy parameter handling
Merge branch 'master' into blendmasks
masks and blending: bit better adaptation of gui elements to mask types used
Merge branch 'master' into blendmasks
masks and blending: revamping of blend modes: normal and unbounded
Merge branch 'master' into blendmasks

Conflicts:
	src/develop/blend.c
	src/develop/blend_gui.c

Fix conflicts introduced by commit 9fbce5ddc31eb7765dd6126853adbc32c86fb9f6.
Learning: we should run the style beautifier only shortly before a release and after master
has been consolidated with all needed side branches.
Merge branch 'master' into blendmasks
masks and blending: add icons for quick access of creating new shapes

this also makes the masks feature more visible in the blending gui