Gst::Pad: added exception handler for callback calls

Sometimes we can't use default glibmm exception handler, because handler
must be registered in the same thread. There is a few methods, where it's
impossible to register handler, because we don't have an access to this
thread.

	* gstreamer/src/pad.ccg: exception handler implementation, call
	  exception handler in catch(...) blocks.
	* gstreamer/src/pad.hg: exception handler method header