Similarly to the various GimpResource select PDB calls, this allows to call a
core dialog in order to choose a drawable which will be returned back to the
calling plug-in.

This new GimpPickableSelect dialog is a subclass of GimpPdbDialog and uses the
same GimpPickableChooser widget as GimpPickablePopup, except that since it's
inter-process window management, it is harder to make a popup positioned
accurately relatively to a parent (especially on Wayland). This is why it's a
separate widget as a simpler dialog (which we will still try to make transient
as much as possible across platforms).