Change the input for grid units/pixels to a simple checkbox toggle from a
dropdown menu.
Add a new specialized AccessorMutator class to support this operation.
Fix code style issues per Victor's review
Add toggle to work in pixel space or grid space, per the issue description.
Each element stores a boolean property that determines whether or not it
snaps to grid space or pixel space. Coordinates are converted between spaces
on toggle, preserving the size of the element in pixels.
To complete: change UI element for toggle to a checkbox.
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.
