GUACAMOLE-1231: Implement basic support for restoring minimized RAIL windows

[necouchman]

This is a really basic update for the Guacamole RAIL channel that handles situations where RAIL windows are minimized by simply immediately sending the SC_RESTORE command for that window back to the RDP server, causing the window to immediately re-appear. This resolves, at a very basic level, the issue with RAIL windows that are minimized becoming inaccessible within the Guacamole interface.

Looking at both the MS-RDPERP specification and the xfreerdp client, I suspect there is a lot more potential for some really cool functionality within the RAIL channel that could be implemented within Guacamole - like the ability to track what RAIL windows are open for a particular client, perhaps even open them in new/separate windows, and perhaps even integrate with the current multi-connection support to provide an icon representation of "minimized" RAIL windows. But, I thought I'd go the simpler route, first, and just solve the problem of not being able to access things that get [mistakenly] minimized.

[mike-jumper]

This is a really basic update for the Guacamole RAIL channel that handles situations where RAIL windows are minimized by simply immediately sending the SC_RESTORE command for that window back to the RDP server, causing the window to immediately re-appear. ...

I rather like this. It immediately mitigates the issue at hand without requiring huge changes, but also doesn't get in the way of some future representation for minimized windows.