make the event loop accessible in App

[jhickner]

If an app needs to run a child process (in my case I wanted to open $EDITOR to edit a file), we need to be able to stop and restart the event loop.

This PR adds a loop field to the vxfw.App struct, so the loop is accessible.
This allows, e.g.

self.loop.?.stop();
editor.edit("file.txt");
try self.loop.?.start();

Without this you can still run a child process and open an editor, but it's unresponsive to commands as they're being consumed by EventLoop's ttyRun thread.

[rockorager]

I've been hesitant to expose the loop directly invxfw. I'd like to let the widgets be usable from other loops, so I've been trying to keep anything like this as a Command that can be added to the CommandList ArrayList passed into the event handlers.

What do you think about adding a suspend and start command into vxfw.Command?