[feat] expose gateway selector in gui

gui/gui.qrc

gui/handlers.cpp

gui/handlers.h

gui/js/maps.js

gui/qml/VpnState.qml

gui/qml/main.qml

pkg/backend/api.go

pkg/backend/status.go

pkg/bitmask/bitmask.go

pkg/vpn/bonafide/bonafide.go

pkg/vpn/bonafide/eip_service.go

pkg/vpn/bonafide/gateways.go

pkg/vpn/openvpn.go