Allow for user resolution setup when starting the container

[stefanoborini]

We want to give the user the ability to set the resolution for vnc containers. This will require

• selection combobox in the web UI that must appear only for VNC images.
• a protocol to communicate this request through the WebAPI.
• delivery of this information to the container, possibly through envvar.
• awareness of the images of this parameter so that the appropriate resolution can be chosen for the X windows server.

[stefanoborini]

Fixed in various PR. #263 #264 the main ones.