Clients::Studio: added "About" window

	* src/clients/studio/mainwindow.cpp: added "About" item signal
	  handler
	* src/clients/studio/mainwindow.h: helper method for reading
	  widgets from builder added
	* src/clients/studio/sigcpp_lambda_hack.h: added workaround which
	  allows use lambda for non-void functions
	* src/clients/studio/ui/mainwindow.glade: added "About" dialog
	  description
	* ui/mainwindow.glade: moved ui file to the right place