Re-organize de-selection so that there is less conflict between tracking
and masking operators.
Still not fully ideal: the LMB selection does not de-select everything
now since the `mask.select` with `deselect_all` is only added to the
keymap when the RMB is the select mouse. While this is sub-optimal, this
seems to be how mask selection behaved in the Image Editor in 3.1.
Not sure it worth looking into a more complete fix, as it will likely be
too big to be safe for a corrective release.
Differential Revision: https://developer.blender.org/D15183
ea39d808b5