[Launcher] Better mapping between favourites, executables and window classes

[JakeStanger]

Is your feature request related to a problem? Please describe.

The launcher favorites option accepts any value that xdg-open takes, but for grouping to work this value has to match that of the window app_id / class.

Describe the solution you'd like

Either the favorites or grouping logic (or both) should be smarter to make it easier to configure. Some well-knowns should be handled without requiring user configuration.

Describe alternatives you've considered

None yet.

Additional context

Discussed in #465.

Properties in the .desktop file such as StartupWMClass may potentially lend a hand.