Add Element class

[mattrasto]

Element data is disconnected from its rendered element, and operating on single elements is difficult. Adding an Element class would provide several benefits:

1. Element data and elements would be held in a single location
2. Publicly-accessible element-specific methods would allow users more control over elemental operations (eg. move(), force(), fix(), neighbors(), etc.)
3. Significant cleanup of code in other objects regarding elemental operations
4. Increased flexibility for user-implemented subelements (eg. additional labels, dialogs, etc.)
5. More precise control over elemental style retainment. Elements can maintain their own state, which could include a stylemap stack, with layers representing styles applied/unapplied over time.