Update preferences ui to select and save gateway.
Add config option to set & get preferred gateway.
Add method to return gateways with their names.

Also the test code in the bottom was updated to work.
Add gateway selection feature to the preferences.
Start the vpn with the user configured gateway.
Add changelog. Closes #3505.
Merge remote-tracking branch 'ivan/feature/3505_preferences-select-gateway-manually' into develop