Implement text dropdown field

[tmickel]

Requirements:

#148

1. Displays text in a list:
   
2. Can display separators as above.
3. Text of selected item is displayed in the block.
4. As an option, text is also editable to arbitrary values in the block:
   
   Perhaps this should actually be implemented as an extension to field_textinput, and non-editable dropdowns should be field_dropdown? Thoughts?
5. If the number of options is too many to fit in the workspace (even after trying to "flip up"), deal with scrolling within the dropdown. It should be scrollable using the mousewheel (e.g., just by using CSS overflow: auto) and also probably have arrows at the top/bottom of the drop-down, which when hovered over, trigger scroll:
   
   Pending further design especially for touch @carljbowman
6. Certain values can trigger call-backs, e.g.:
   
7. Values can be populated with a call-back, e.g., in this case, it includes a list of sprites:
   

[tmickel]

Implemented using DropDownDiv in #407.

Separator should already be implemented. Need final behavior for coloring in the block, icon for the field, flipping the icon when rendered to secondary position, and adding callbacks for items like "new message..."

And thinking more deeply about scroll behavior and touch.

[tmickel]

Breaking into more granular issues for future milestones.