evas: improve logic regarding events filtering enablement

evas: improve logic regarding events filtering enablement

Only the size of events_whitelist isn't enough, because in some
cases the user may be disabling the usage of a specific seat.

Considering the following scenario, the issue will easy to understand:
 - an application with two entries (one to be used by seat 1 and other
     by seat 2)
 - the first seat is announced - it is enabled for entry 1 and
     disabled for entry 2
 - the second seat is announced

Before second seat is announced, the first seat would be able
to input the entry 1, because the events_whitelist of such
object will continue empty.

So a flag will be used to identify an object with active
filter.

Reviewed By: iscaro

Differential Revision: https://phab.enlightenment.org/D4498