Implement different types of Select Opaque action

Now we have four different actions:

* Select Opaque --- replaces the current selection
* Select Opaque (Add) --- adds to the current selection
* Select Opaque (Subtract) --- Subtracts from the current selection
* Select Opaque (Intersect) --- Intersects with the current selection


BUG:346892
CC:kimageshop@kde.org