Switched to GTK+3.6 threading model, make pthreads compulsory.
Merged #385
Tested and merged pull request #367 - Fixed GTK3 issue
Merge branch 'threading2' of github.com:giox069/Remmina into giox069-threading2
Switched to GTK+3.6 threading model, make pthreads compulsory. Merged from 'giox069-threading2' into next
GTK3 migration, fixes #365