Remove some Q_OBJECT macros (again)

It looks like I can't take out the one in EvalDatum -- it will
compile file, but then crash upon construction of a ShapeDatum
(for unknown reasons, in a strange way)
Tab key changes focus (but doesn't yet cycle)
Move getCanvas out of slots section of header
Return key closes window
Shift-tab cycles backwards through datums
Merge branch 'tab-changes-focus'