Core: implemented methods for logging exceptions

      * configure.ac: checking for glibmm-2.4
      * src/core/Makefile.am: added glibmm cflags
      * src/core/logger.{cpp|h}: implemented specified methods
Clients::Studio: used logger in gstreamer-studio app

	* Makefile.am: change build order (core library first)
	* src/clients/studio/Makefile.am: include and link core library
	* src/clients/studio/gstreamer-studio.cpp: catching exception on
	  a application's top level
	* src/clients/studio/mainwindow.cpp: logging build exceptions,
	  closing application fixed
	* src/clients/studio/ui//mainwindow.glade: renamed quit menu item