XBlocks can disable navigation chrome.

@nedbat @cpennington There is an option to:
* Enable/disable accordion navigation chrome
* Enable/disable/repoint tab navigation chrome

This allows for full-screen XBlocks (e.g. a code IDE, or large video
player). It is also the first pass at allowing top-level XBlocks. It's
also now possible to make a chromeless XBlock, point a tab to it, and
make it point back to that tab.

Next steps down that path would be:
* Fix up how tabs are handled. The current version is a hack.
* Create appropriate XBlocks for courseware, tabbed navigation,
  etc. to reach feature parity
* Invert/rejigger the XML format.
Changed. Two side notes: 

1) The same design pattern is used a few lines down: 

% if masquerade is not UNDEFINED:

2) I *believe* mako does something clever where it treats undefined as false in many cases. I've seen that work. I'm not sure what the rules are, and I'd rather not rely on magic.