Refactor expense commands, update tests, UI, DG

[jamessspanggg]

• refactor add, edit, delete, clear, list to addexpense, editexpense, deleteexpense, clearexpense, listexpense
• add tests to expense to increase coverage
• update browser panel to display selected expenses
• update browser panel to display categories in terms of images
• update expense list to always display expenses in descending order of date
• refactor tests based on the change in expense listing
• update undo/redo implementation in DG

[jhocx]

Thank you! These changes are very useful, makes things clearer too.