Revert "location-source: Make start/stop explicit"
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
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