cmd/systray: add a basic linux systray app

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>

go.mod

@@ -4,10 +4,12 @@ go 1.23
 
 require (
 	filippo.io/mkcert v1.4.4
+	fyne.io/systray v1.11.0
 	github.com/akutz/memconn v0.1.0
 	github.com/alexbrainman/sspi v0.0.0-20231016080023-1a75b4708caa
 	github.com/andybalholm/brotli v1.1.0
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be
+	github.com/atotto/clipboard v0.1.4
 	github.com/aws/aws-sdk-go-v2 v1.24.1
 	github.com/aws/aws-sdk-go-v2/config v1.26.5
 	github.com/aws/aws-sdk-go-v2/feature/s3/manager v1.11.64
@@ -28,6 +30,7 @@ require (
 	github.com/dsnet/try v0.0.3
 	github.com/elastic/crd-ref-docs v0.0.12
 	github.com/evanw/esbuild v0.19.11
+	github.com/fogleman/gg v1.3.0
 	github.com/frankban/quicktest v1.14.6
 	github.com/fxamacker/cbor/v2 v2.6.0
 	github.com/gaissmai/bart v0.11.1
@@ -130,6 +133,7 @@ require (
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/gobuffalo/flect v1.0.2 // indirect
 	github.com/goccy/go-yaml v1.12.0 // indirect
+	github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0 // indirect
 	github.com/google/gnostic-models v0.6.9-0.20230804172637-c7be7c783f49 // indirect
 	github.com/google/pprof v0.0.0-20240409012703-83162a5b38cd // indirect
 	github.com/gorilla/securecookie v1.1.2 // indirect