no fullscreen-screensharing with muliple participants

[mahibi]

Steps to reproduce

1. start a call with multiple participants
2. share screen from one user

Expected behaviour

• screenshare is fullscreen

• possibility to close fullscreen by "X" icon

• add some indicator in gridview to see who is sharing the screen

• manually open this screenshare by clicking

• also fix #913 to seen the whole screen that is shared

Actual behaviour

screenshare is not fullscreen