plugins: compositor: modify ScreenShooter to accept file name

Feature #731.
Signed-off-by: Nikolay Nizov <nizovn@gmail.com>
plugins: compositor: ScreenShooter: return only file name

Signed-off-by: Nikolay Nizov <nizovn@gmail.com>
Merge pull request #89 from nizovn/feature731

plugins: compositor: modify ScreenShooter to accept file name