[LinuxCNC/linuxcnc PR#446] Replace `python-notify` with our own PyQt5 DBus system notification module

python-notify when used with Qt5 on Debian 9 causes gtk errors which prevent QtScreen from starting. This replaces that package with a pure Qt module that sends requests over DBus to the system notification server.

Depends on:
* python-dbus
* python-dbus.mainloop.pyqt5 (optional)

If python-dbus.mainloop.pyqt5 is not installed any notification actions with callbacks will not work, but the notifications will still be shown.

Tested on:
* Debian 9
* Ubuntu 18.04