Cleanup: Animation, simplify channel selection code

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.

source/blender/editors/include/ED_anim_api.h

@@ -620,9 +620,11 @@ void ANIM_flush_setting_anim_channels(bAnimContext *ac,
                                       eAnimChannel_Settings setting,
                                       eAnimChannels_SetFlag mode);
 
-/* Deselect all animation channels */
-void ANIM_deselect_anim_channels(
-    bAnimContext *ac, void *data, eAnimCont_Types datatype, bool test, eAnimChannels_SetFlag sel);
+/* Select or deselect animation channels */
+void ANIM_anim_channels_select_set(bAnimContext *ac, eAnimChannels_SetFlag sel);
+
+/* Toggle selection of animation channels */
+void ANIM_anim_channels_select_toggle(bAnimContext *ac);
 
 /* Set the 'active' channel of type channel_type, in the given action */
 void ANIM_set_active_channel(bAnimContext *ac,