Make GladePlaceholder a scrollable widget (hypothetically) to avoid runtime warnings.

	* plugins/gtk+/gtk+.xml.in: Disabled "has-separator" of GtkMessageDialog as it ignores the separator
	  and the setting is useless (not to mention setting it causes runtime warnings: fixes bug 587288).

	* gladeui/glade-project.[ch]: Expose glade_project_get_target_version()

	* plugins/gtk+/glade-gtk.c:
	  - check project target gtk+ version to decide initial state
	    of GtkEntry::buffer.
	  - Substitute the old manual evaluation with GPC_VERSION_CHECK()
	  - Avoid critical warnings when setting GtkColorButton properties to NULL
	  - Avoid critical warnings when setting GtkComboBoxEntry::text-column < 0

	* gladeui/glade-property-class.h: Created convenience macro GPC_VERSION_CHECK

	* gladeui/glade-utils.c: Allow loading of libraries installed in optional non-system prefixes;
	  (so that GtkSourceView catalog is buildable and usable from your jhbuild environment)
	  assume a path that is: ${prefix}/lib/glade3/modules/../../ and search it after the initial
	  module search path.