Fix crash in VC when adding widgets.
I added Qt::QueuedConnection to some connect() calls. I don't understand the mechanism, it seems to work.
Please review and/or extend to other menu actions.

I got the idea from https://bugs.kde.org/show_bug.cgi?id=311167

Fixes #120.
Merge pull request #146 from janosvitok/issue120

Fix crash in VC when adding widgets.