This adds a systray app for linux, similar to the apps for macOS and
windows. There are already a number of community-developed systray apps,
but most of them are either long abandoned, are built for a specific
desktop environment, or simply wrap the tailscale CLI.
This uses fyne.io/systray (a fork of github.com/getlantern/systray)
which uses newer D-Bus specifications to render the tray icon and menu.
This results in a pretty broad support for modern desktop environments.
This initial commit lacks a number of features like profile switching,
device listing, and exit node selection. This is really focused on the
application structure, the interaction with LocalAPI, and some system
integration pieces like the app icon, notifications, and the clipboard.
Updates #1708
Signed-off-by: Will Norris <will@tailscale.com>
Will Norris