Only one mscolab main window should be allowed

[Joern Ungermann]

It is currently possible to open multiple mscolab windows. To a certain extent this works. Still as mainwindow for future MSS, a single one is sufficient as one can simple open a separate process to connect to multiple servers.

‌

‌

[Reimar Bauer]

Allowed only one mscolab instance, fixes issue #504 and issue #506

→ <<cset 06cfb1175b84>>

[Reimar Bauer]

Merged in tanish19/mss/single_mscolab_instance (pull request #822)

Allowed only one mscolab instance, fixes issue #504 and issue #506

→ <<cset 6e329a740784>>