Started adding control functionality to the GUI.

- Controller.cc/.h: implemented thread-safe message loop.
- GuiController.cpp/.h: implemented thread-safe wrapper between the controller and the GUI.
- MainWindow.cpp/.h: moved signals to GuiController; using controller to work with a VoltageSource.
- MainWindow.ui: fixed to be compatible with QT 4 and "good looking" on Debian.
- VoltageSourceControl.pro: fixed to be compilable on Debian.
- exception.h: added constructor by default to be compatible with QT message queue.
- main.cpp: using GuiController.