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/space_graph/graph_select.c

@@ -1471,7 +1471,7 @@ static int mouse_graph_keys(bAnimContext *ac,
      * doesn't depend on this
      */
     if ((sipo->flag & SIPO_SELCUVERTSONLY) == 0) {
-      ANIM_deselect_anim_channels(ac, ac->data, ac->datatype, 0, ACHANNEL_SETFLAG_CLEAR);
+      ANIM_anim_channels_select_set(ac, ACHANNEL_SETFLAG_CLEAR);
     }
   }