Replace "plugin system" with a simple and more maintainable solution

[buhtz]

Problem

• The "plugin system" add extra complexity without benefit.
• It blocks migration to modern Python build system (Migrate to Python build-system (PEP 517 & PEP 621) #1575).
• To my knowledge there are no user created plugins in use by anyone.
• It is not a real "plugin system" but more an event dispatch or hook system.

Solution

Replace the functionality of the existing plugins with in-build solutions.

More details

The following plugins do exist:

1. common/plugins/usercallbackplugin.py
2. qt/plugins/notifyplugin.py
3. qt/plugins/systrayiconplugin.py

It is not hard to imagine alternative solutions to those three components without loosing their features. The user-defined user-callback scripts will work and won't broke.