[pkg] add temporary vendoring of getlantern/systray to fix win bug

vendor.json

vendor/README

vendor/github.com/getlantern/systray/.gitignore

vendor/github.com/getlantern/systray/LICENSE

vendor/github.com/getlantern/systray/README.md

vendor/github.com/getlantern/systray/systray.go

vendor/github.com/getlantern/systray/systray.h

vendor/github.com/getlantern/systray/systray_darwin.m

vendor/github.com/getlantern/systray/systray_linux.c

vendor/github.com/getlantern/systray/systray_nonwindows.go

vendor/github.com/getlantern/systray/systray_windows.go

vendor/github.com/getlantern/systray/systray_windows_test.go