Implement Ctrl+click shortcuts for selecting opaque pixels on a layer
Now one can click on a layer's thumbnail in the Layers docker to
have all the opaque pixels selected.
- ctrl+click --- replace selection
- ctrl+shift+click --- add selection
- ctrl+alt+click --- subtract selection
- ctrl+shift+alt+click --- intersect selection
CC:kimageshop@kde.org
CCBUG:346892