These actions raise a GimpViewableDialog. For this to work, I made this
widget work with a list of GimpViewable, not a single viewable anymore
(so maybe the widget class name should change?).
When this list contains only a single GimpViewable, it will display
exactly like before, with a viewable preview. With several viewables,
the preview won't show.
This allows to add masks to all selected layers at once, with the same
basic options for all masks, as set in the dialog.
Try to sort all GIMP_ICON_* defines into FDO categories like in
https://specifications.freedesktop.org/icon-naming-spec/latest/ar01s04.html
Add defines for all icons we override, rename some icons to their FDO
standard names, and mark the ones we duplicate with a comment so we
don't forget to rename those to standard names in 3.0.
Nobody has them anymore, and they are deprecated in GTK+ 3.x. This
also fixes all conflicting mnemonics except those I missed, but we can
fix them now.
