manager: pass ECalObjModType to update and remove

Currently, GcalManager always assumes E_CAL_OBJ_MOD_THIS
when modifying or deleting events. Since we're landing
the groundwork for recurrence management, we can't assume
this anymore.

This patch, then, allows GcalManager:modify_event() and
:remove_event() receive the ECalObjModType instead of
assuming E_CAL_OBJ_MOD_THIS. However, this commit does
not add recurrence support yet, and every call to these
functions are still passing E_CAL_OBJ_MOD_THIS.

https://bugzilla.gnome.org/show_bug.cgi?id=782755