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 from master in 2 minutes and 16 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #72458
ubuntu-16.04

00:01:49

passed #72459
ubuntu-17.10

00:01:54

passed #72460
ubuntu-18.04

00:02:14