Make window widgets that blocks click propagation #1

You can currently click through the "About" window in the example to hit any button underneath it.

We'll want to add something like a vxim.window(id, .{ .x, .y, .title}) widget that is part of the focus chain to make it so you can't click through the window. The widget should return the "body" part of the window that you're allowed to write to. The window should also clear the body to make sure nothing is rendered through it.