Add support for filtering/mocking method calls

[hadess]

If there was an externally manipulatable API in xdg-dbus-proxy to add support for filtering or mock APIs, it would be possible to have, for example, xdg-desktop-portal block access to NetworkManager unless location services were authorised for this app, mimicking the functionality in iOS:

An app linked against iOS 12 or earlier receives a dictionary with pseudo-values. In this case, the SSID is Wi-Fi (or WLAN in the China region), and the BSSID is 00:00:00:00:00:00.

This could also be used to:

• filter responses from Avahi to only show services that the app has declared caring about
• make it impossible to scan or manipulate unpaired devices for bluez
  etc.