Place newly added ValueView below last added view
Begin ValueView meta-key special processing

When dragging a ValueView with the meta-key pressed, it will
increment (or decrement) its value based on the drag offset.

The communication mechanism for this is set up in this commit.
Still to do is the actual mutation of the ValueView's Value. For
that, we'll need to add a few methods to Value for this to work.
Make Value mutable

Debated whether or not to extract out a MutableValue object, but
in the end decided that it's a tad too hard to do correctly in
JavaScript, but also, it's not really necessary (at this point,
at least).
Make ValueView value adjustable via mouse