efl/player: merge start+stop methods into 'playing' property

efl/player: merge start+stop methods into 'playing' property

this has some overlap with the existing 'play' property which will
soon be renamed. the intent here is that there is a property for
controlling the 'playing' state and then another property for managing
'pausing' the play state

ref T7877

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D10113