[FIX] web: correctly init searchview when no action_id

the method prepare_dropdown_menu wasn't called when the searchview did
not have an action id.  This can cause problems in some cases (for
example, a crash in messaging when toggling the favorite menu)
[REF] mail: _track update.

_track is not a method that returns the subtype to trigger in a given
tracking context. A tracking will now lead to only one subtype
being trigerred, instead of having multiple possible subtypes.

Now, one tracking leads to one message with zero or one subtype.
This simplifies the model for future evolution of chatter and mail.