Fix various problems in the Action Group rearranging code. All fixes are
necessary to resolve the bug.
- Before groups are rearranged, the channels are moved into their
respective groups (so no longer referenced by `action->channels`). A
temporary group is made for ungrouped channels. The code made
assumptions about the channels being in the same order as the groups;
that assumption has been removed.
- Looping over channels in an Action Group should stop when reaching the
last channel, and not until `NULL`.
- After all the reshuffling is done, the `action->channels` linked list
wasn't terminated properly. Now `first.prev` and `last.next` are set
to `NULL` to avoid infinite loops.
8c74c35ecb