test-widget: remove basic search and replace feature

See test-search for the search and replace.
test-widget: remove TEST_XML_MEM

The GtkSourceView website says:

	We have not an official tutorial, but the source of test-widget
	program is a good starting point for learning how to use the
	GtkSourceView API.

So testing the xml memory in test-widget is not really a good place. If
that's really needed, another test program can be written.
test-widget: clean gtk_source_buffer_load_file()
test-widget: improve code for opening a file
test-widget: continue the clean-up
Deprecate the GtkSourceMarkAttributes:stock-id property

Deprecate also the getter and setter.

GtkStock is deprecated.