Gns 44 - Send heartbeats from GUI to server

The GUI will now send heartbeat messages to the server every heartbeat_freq milliseconds.  It does this for both local and remote servers.  While not needed for the local server case, it does make it easier while we are developing the feature.

To test this setting via the config file, open ~/.config/GNS3/GNS3.conf and in the [Servers] section, add

heartbeat_freq=5000

This will cause heartbeat messages to be sent every 5 seconds