cmd/systray: set app icon and title consistently

Refactor code to set app icon and title as part of rebuild, rather than
separately in eventLoop. This fixes several cases where they weren't
getting updated properly. This change also makes use of the new exit
node icons.

Updates #1708

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>
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