[UI] Provide a checkbox for enabling console sharing

The checkbox is located in the Advanced preference page of Saros.
The whole console sharing feature only works if the box is checked.

Change-Id: I1cc53cdc418a2111b55473d36ea80b410c8f66fa
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1457
Tested-by: Jenkins CI
Reviewed-by: Andrej Szaffranietz <a.szaffranietz@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>