This basically reverts commit 88d42b44
but had to be heavily rebased/changed.

Since all actual location sources are now singletons, one fundamental new
change is that each source now keeps a count of start requests and don't
actually start or stop until this count reaches 1 or 0, respectively.

Conflicts:
	src/gclue-locator.c
	src/gclue-service-client.c
	src/gclue-web-source.c
	src/gclue-wifi.c