Basic UI glade file.

https://github.comca/gst-switch/issues/50

This is the 2nd time I have ever used glade, so no idea how useable this really is.
If it is useable I would really like to see it used to drive the server.
My goal was to have some UI element that each keyboard command can be bound to.

I didn't try to put them on the current UI, I think a separate window will be fine.  
There are quite a lot of buttons, mainly the 3 sets up arrows with shift states.

I suspect the UI elements should be named something better than button1, but I don't think that should be a blocker. 
I also suspect the up/down/left/right buttons should be in a diamond configuration, or inverted T like the arrow keys on a keyboard.  again, not a blocker.