Fix systray icon & popover issues

Improvements for all users:

• We added more traces and test cases to help us fix popover positioning issues.
  Please send us a message from the app in case you encounter an issue, so we
  can get your traces and understand your specific case.

Improvements for macOS users:

• Make sure the popover at least shows up to the top right when the reported
  tray icon position is broken (as it seems to happen from time to time).

Improvements for GNOME 3 + Wayland users:

• Show the popover near the top icon, not at the bottom of the screen.

Improvements for KDE users:

• Fix issue preventing the popover to show up when clicking the systray icon
  on KDE (@hleroy)

Improvements for developers & contributors:

• Use unique user agent per client instance, so we can know how many people are
  actually using it.
• Instructions to set up a development environment on Ubuntu (@hleroy)

See also known issues.

Happy syncing!