I've gone ahead and added the BaseMnemonic class, along with an IMnemonic interface that all CPU mnemonics (instructions, op-codes - whatever you want to call them) inherit from.
The past hour or so has been spent improving the interface and adding things where necessary. The main window now has the standard set of root menu items (File, Edit, View, Debug, Tools, Help) and there's a (currently collapsed) project viewer.
Added a Messages window that will show To-do items, syntax errors, and the like. Also added Status Register to the Registers window. The tooltip for each register's value also displays the value of that register's contents in base 10 (Decimal) and likely base 16 (Hex) in the near future, too.
Next up is making use of the BaseMnemonic class by implementing the
Move instruction. Oooh!