dbus,manager: Add DeleteClient() method

Add a method for apps to explicitly delete their client objects. This is
useful for long-running applications (e.g gnome-settings-daemon) for
freeing their associated resources while they are not using Geoclue. This
will also allow geoclue to exit when it's not actually being used by these
applications.

Relevant: https://gitlab.gnome.org/GNOME/gnome-settings-daemon/issues/127

Fixes #99.
3 jobs for client-teardown in 2 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #72453
ubuntu-16.04

00:01:51

passed #72454
ubuntu-17.10

00:01:56

passed #72455
ubuntu-18.04

00:02:22