Callbacks: Replace with Events

[avose]

The callbacks that are passed around from the root window down to lower level widgets are getting out of hand. These should all be replaced with events, and the parents should bind these events if they are interested in them. This would clean the code up a lot.

[avose]

There is now a glsEvents.py file with custom events for GLShell. These are in use for a few things, but some callbacks still need to be replaced with events.

Also, there are some "watcher" patterns using wx.CallAfter() that can probably be replaced with events.

[avose]

Most of the callbacks in the terminal notebooks have been replaced by new events.

I was also able to remove one of the "watcher" patterns using wx.CallLater() continuously with a single event dispatch. This in particular is nice, as it will reduce the load on the main GUI thread, and allow scaling to a higher number of open terminals.

[avose]

Also, a large part of all the craziness I coded up was because I didn't see a cleaner way around the issue of a wxPython object being called when it's C++ object was destroyed. There were all kinds of issues with trying to have a widget delete itself, only to cause an error because the C++ object wasn't around anymore. However, a nice trick one can do here is to just test self or the Python object with an if statement. The object will test equivalent to False if the C++ object has been destroyed.