Because sub-items of groups can no longer reroute their signals to the
parent scenes of groups, delegates removal handling to SourceTreeItem
instead of OBSBasic.
(This commit also modifies UI)
Changes groups to their own independent type, "group". This allows them
to be used like other regular types, and allows the ability to reference
groups in multiple scenes. Before, a group would always be linked to
the scene it was in. This made it cumbersome for users to modify groups
if they had a similar group in multiple scenes (they would have to
modify each group in each scene). Making groups like other source types
makes more sense to solve this issue so they can be referenced in
multiple scenes at once. This also removes a significant amount of
group-specific handling code required for implementing groups in the
front-end.
One limitation however: due to the way sub-items of groups are
seamlessly modifiable and sortable as part of the whole scene, the user
cannot have multiple references to the same group within one scene.
(This commit also modifies UI)
Removes obs_scene::group_sceneitem and replaces it with
obs_scene::is_group. Changes a number of other functions related to
groups so that a group is not inherently tied to a specific scene, and
helps allow a single group to be referenced in multiple scenes if
desired.
