feat!: register Keysmith with D-Bus

This change provides the most basic D-Bus support in Keysmith.
At start up, Keysmith will now register itself and abort if another
instance is already running. Full support for the xdg D-Bus activation
specification (such as URI opening) is not yet implemented.

By default this feature is disabled on Android, but enabled on 'all'
other platforms. Explicit control may be exercised by running CMake
with -DBUILD_DBUS_INTERFACE=<ON|OFF>.

Issues: #18