Add screen selection in screen grab widget

Fix #243 (closed)

I basically moved the combobox code to mainwindow.cpp, where the other controls are created. I then plugged the screen setting signals to RecManager. It is not perfect because I have to pass by Monitor, but that's a legacy of previous refactoring and there is more to do in this area.

In the end it works, I am now able to select which screen to grab.