Replace FvwmTabs' tabber_client.py with FvwmMFL

The new FvwmTabs has rewritten the classic Perl FVWM tabbing module in Python/Tk under the same name FvwmTabs. Everything works as expected—multiple tabbers, assigned tabber IDs, the right-click menu,... (Budget: €20 - €25 EUR, Jobs: Debugging, Linux, Python, Scripting)