remixerView: nest summaryView in permanent div
shell: remove _attributeCanBeEmpty bitmap

for now, don't permit empty string values accept as the default value
shells: move default attribute management to model
editSummaryView: fade default text in input fields
editSummaryView: transfer non-default values

when model is changed, transfer the old model's non-default values to
the new model
tests: delete tests that use setTimeout

Expectations wrapped in setTimeout don't register within their suite. As
far as I know, there is no way to test that something happens when the
call stack clears (i.e. happens on `setTimeout(fn, 0)`).
editSummaryView: add placeholder text
remixerView: bugfix: hide summaryView in lastchild
elapsedTimeControl: widen a bit, hide overflow