native engine expose VNC server, if enabled

[jonasfj]

Let's add a configuration option to the native engine.. Such that it can be configured to connect to a VNC server on localhost and use this to expose display.

This way when installing tc-worker with native engine you just need to install a VNC server on localhost and set the engine configuration to use it, and enable the interactive plugin, then you have interactive display.

The implementation is rather easy... just returns a list of zero or one display, the VNC testing code already contains logic to extract screen resolution which could be reused, or this feature could be omitted since it's not critical. Then the actual thing is returning a pipe from OpenDisplay and copy from the pipe to a connection to the vnc server on localhost.

Note: this requires installing a vnc server on localhost, but it's a nice way to do it, as such vnc servers are readily available on most platforms. There might be some things to consider with respect to the per-task user feature...