SPICE connection through proxy

Problem to solve

Remmina should provide support for SPICE connections through a proxy.

Further details

The SPICE-specific remote-viewer supports connecting through a proxy. This is a great feature for providing access to remote desktops through regular HTTP(S) connections.

As the connection "logic" is implemented in the spice gtk library already, it should be suffient to add a tab to the connection properties and a switch/flag to the command line invocation and forward the information.