Add RPC command "togglenetwork" to toggle network activity temporarily.

RPC command "togglenetwork" toggles network and returns new state after command.
RPC command "getinfo" returns "networkactive" field in output.
Add gui feedback and control of network activity state.

Add getNetworkActive()/setNetworkActive() method to client model.
Send network active status through NotifyNetworkActiveChanged.
Indicate in tool tip of gui status bar network indicator whether network activity is disabled.
Indicate in debug window whether network activity is disabled and add button to allow user to toggle network activity state.