Split up `ANIM_deselect_anim_channels()` into
`ANIM_anim_channels_select_set()` and
`ANIM_anim_channels_select_toggle()`.
`ANIM_anim_channels_select_set()` is equivalent to the old
`ANIM_deselect_anim_channels(..., false, ACHANNEL_SETFLAG_xxx)`.
`ANIM_anim_channels_select_toggle()` is equivalent to the old
`ANIM_deselect_anim_channels(..., true, ACHANNEL_SETFLAG_ADD)`.
`ANIM_deselect_anim_channels(..., true, ACHANNEL_SETFLAG_CLEAR)` was
also called once. The `true` parameter suggested the "toggle" behaviour
was intended, but the `ACHANNEL_SETFLAG_CLEAR` argument prevented any
toggling. This is now replaced with `ANIM_anim_channels_select_set(ac,
ACHANNEL_SETFLAG_CLEAR)` to make this explicit.
No functional changes, just a cleanup in order to better understand how
the selection works.
78d1aa4d52