Migrate all the D-Bus calls to GDBus.

Given that the python-dbus interface only receives fixes and is considered by
its author and maintainer as un-pythonic and is not recommended by him, it seems
more logical to use another, more recommendable interface.

As gatt-python already makes use of PyGI and GLib, it makes sense to use GDBus which
comes as a part of GLib.

This also effectively removes the python-dbus dependency.

README.md

@@ -66,11 +66,10 @@ BlueZ also provides an interactive commandline tool to interact with Bluetooth d
 
 ### Installing GATT SDK for Python
 
-To install this GATT module and the Python3 D-Bus dependency globally, run:
+To install this GATT module globally, run:
 
 ```
 sudo pip3 install gatt
-sudo apt-get install python3-dbus
 ```
 
 #### Running the GATT control script