efl/player: rename 'play' property to 'pause'

efl/player: rename 'play' property to 'pause'

this is a bit of an overhaul wherein the existing 'play' mechanics are
all inverted. 'pause' is a state which stops playback but does not affect
the playback_position property.

this patch also includes implementations of Efl.Player::playing for
a couple classes which (now) only implement pause, as this is a requirement
for the objects to actually activate their animations

test cases:
* unit tests
* all elm_test animation cases
* elm_test video
* rage

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D10114