Add support for dynamic image sizing
Add image size util (required for image resizing)
Add image dropdown field
Convert FieldImageDropdown to inherit directly from Field. Always use rectangle overlay for image dropdown click.
Change image dropdown to use arrow as expansion widget
Use constants and public helpers for block dragging status
Constructor comment with more detail about dropdown input format
Remove commented CSS
Remove unused params
Explicitly set position to relative (dashboard overrides higher-level goog ui menu class)
Merge pull request #68 from code-dot-org/image_dropdown_fields

Add support for image dropdown fields (WIP-ish)