cmd/systray: properly set tooltip on different platforms

On Linux, systray.SetTitle actually seems to set the tooltip on all
desktops I've tested on.  But on macOS, it actually does set a title
that is always displayed in the systray area next to the icon. This
change should properly set the tooltip across platforms.

Updates #1708

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f