qtgui: fixed thread-safety in GRC variable-type blocks

Partial bugfix for thread-safety issues in qtgui. Variable-type blocks fixed only.
_Analysis showed potential problems in sinks also. I tried to fix that too, but I confused with complex code and many dead/outdated fragments. Discussion in mailing list: "Bug: gr-qtgui: wrong design of variable-type blocks for GRC (category /GUI Widgets/QT)"..._