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