After reviewing how actions become menu entries in our source code with @rebornix, we decided the naming is too confusing to keep straight. This PR is a large refactor that accomplishes the following:

1. Rename IActionItem and all subsequent classes/interfaces to IActionViewItem. Action items are the 3rd step from Item -> item action -> action item. That transition is confusing but these objects represent how and action should be rendered and run from the UI so ActionViewItem seems appropriate.
2. Update all variable names and comments using the above convention
3. Remove DiffMenuItemActionItem: This is a duplicate of ContextMenu...Item
4. Rename MenuItemActionItem => MenuEntryActionViewItem. View is added because of above. Item is changed to entry as having Item in the name twice is confusing. Do the same for ContextAwareMenuItem...

I would like to follow this refactor with some merging/restructuring of the Menu-specific action/items/viewitems to make it easier to understand. e.g. ContextAwareMenuEntryActionViewItems are used for Toolbars which are different from Menus.