Provide ribbon xml fragments from extensions

[johntyree]

With the addition of the extension loading machinery, we are in a good position to allow PyXLL extensions to be easily deployed and discovered with no manual editing of config files.

What's missing from this is a way to contribute to the ribbon. One can imagine the plugin loading code might also receive an xml fragment from each plugin.

I don't know what the ribbon manipulating code looks like right now, but in the worse case scenario, we could write these ribbon fragments out into a single file and then load it the way we already do. The best case would be something like responding to ribbon update events from the plugins and handling it dynamically.

[tonyroberts]

Sounds good. Let's catch up after the 3.0 release and see what can be done on the pyxll side to make that work best.

[johntyree]

Sure. Depending on how far away that is, I might not be available for a little while. At the very least, I'll leave an open PR here that implements my idea as best we can given what we already have.