Added individual property inputs to the toolbar for height, width, x,
y, and line endpoint coordinates in fixed/bundle.js.
Added "edit<Property>" AccessorMutators for height and width to each
of the element proxies, so these properties can be exposed only for
elements where it makes sense (e.g. boxes, but not lines, since lines
are better controlled by endpoint coordinates).
Added a method "checkNumeric" to ElementProxy.js , to be used
to restrict inputs to integral values for position and size
textfield inputs.
Current issues: endpoint coordinate inputs have undexpected behvaior,
handles disappear after using the input fields to modify element
properties.
