Various Improvements to GorillaEditor

# Fixes
- Undo and redo now work
- inserting and deleting now keeps the GenBank object consistent

# modifications
- Move event handling out of the hands of contenteditable

The only things that contenteditable are providing are:  
1. Allowing focus for the keyboard
2. Displaying a visible caret
3. Allow moving of caret using arrow keys

All other events are handled early by GorillaEditor.