Xwidgets support

[rapgenic]

Implement support for the xwidgets library.

Important points:

• observing properties
• avoid code duplication (dynamic access to properties?)

The output widget is particularly interesting. Should be easy to redirect the default output to the current output widget. A possibility is to use a syntax similar to this:

o = xoctave.widgets.output()
output o           % this could even be implicit
disp("Output")
output default     % or
output             % to restore the default output

Output should be restored to the default on every new cell