Add Systray (AppIndicator DBus spec) module

[andydotxyz]

Starting Systray support - works with Fyne apps but a few things still to do

TODO

• [x] Support submenus
• [x] Fix Slack (and some libappindicator apps missing menus)
• [ ] Find a way to add tests
• [x] Support minibar if it gets merged (1 column instead of 5?)

Fixes #52

Checklist:

• [ ] Tests included.
• [x] Lint and formatter run with no errors.
• [x] Tests all pass.