[ALOY-822] Add support for unsetting properties in TSS via 'undefined'

I would like to be able to *un*set properties via TSS (and XML).

Use case

I want all windows to be heavyweight by default so my app.tss has:

"Window": {
  modal: false
}

But I have one widget which absolutely needs its window to be lightweight. Right now there is no way to do this. I can set modal to a different value like true, but even setting it to null won't make the window lightweight again. I tried setting modal to undefined but it throws a syntax error:

Expected "Alloy", "L", "Ti", "Titanium", "WPATH", "[", "false", "null", "true", "{", comment, end of line, number, string or whitespace but "u" found.

Proposed solution

My suggestion is to accept undefined as a valid value, but instead of actually setting it, use it to trigger removing the property. That way, for that specific window I can set modal to undefined which would remove that property from the dictionary passed to Ti.UI.createWindow.