Allow disabling all sources on compile time, small fixes

Adds meson config options wifi-source and static-source, which allow disabling these sources during compilation.

Removes the addition of default wifi / Ichnaea GeoIP source to the Locator sources list, so when the wifi source is disabled, it is really disabled. Geoclue crashes if the Locator does not have real location sources, so add a trivial GClueDefaultSource singleton which does nothing, but makes the rest of the code work.

Print a warning if a source disabled on compile-time is enabled in configuration.

Also move the small fix commits from !186 to here.