Also fix a bit of "Since:" annotations, some minor spacing fixing and
remove one useless test (if we test if a type is an item, we don't need
to test the children drawable type).
I am using the same GimpDrawableChooser with an additional drawable_type
argument to only show the appropriate tab if we want to limit what can be
chosen.
None of our plug-ins actually use a GimpLayer or GimpChannel only arg so far,
but if we have some day, or if some third-party plug-ins want to have such arg,
now they quite easily can!
