Make 'return' key handled more explicitly in Keyboard...

rather than being just 'ctrl j', which it is, but that's not so useful.
Make LineBuffer handle 'return' by default...

clears self and returns the buffer on 'return'
Make container elements distpatch input to their active containee
Tidy up unused code in test_container_elements
Split up container element tests
Add replace_element method to container elements...

plus some tests
Comment out debug print...

I'm being lazy, but it's useful right now :-)
Rename container._active_element -> container.active_element...

It currently feels like this should be accessible by Junction clients.
Change draw-on-change logic of containers...

They need to do a full draw of their containees at the moment, because the
newly added element may never have been drawn before, and because the layout
may completely change if the number of elements has changed.
Augment callback mechanism of LineInput element
Made container element input handling more robust
Made SplitContainer size constraints overridable
Add default height constraints to Label and LineInput elements
Add pass through of unhandled input data to LineInput
Make container elements blank their draw area before drawing...

This may not be the best way to do this, but currently we leave any area
untouched by the containees drawing completely untouched, which leads to
confusing output.