Adds a new outliner display mode "Collections" which draws the active
collection. We might want to rename it to "Active Collection" if we don't
plan to support showing other collections there.
Also added the buttons for restricting visibility and selectability.
@dfelinto, code in restrictbutton_collection_hide_cb and
restrictbutton_collection_hide_select_cb is duplicated from
rna_LayerCollection_hide_update and
rna_LayerCollection_hide_select_update, maybe utility functions would be
handy for this?
1bcf199d2e