Fix GCM problems with Polling service for incoming deliveries

[ChristianeMussmacher]

• implementation of polling service
• dialogue for automatic activation when GCM service not available
• setting for polling activation
• setting for polling interval (5 / 10 / 30 min)
• no automatic restart of service after reboot system

[elRadish]

Can't be finally tested before #1066 is ready

[elRadish]

Reviewed with @j3nsm4n, merged in 4396a32.