Serious improvements to "open model" experience

Now, only models belonging to the current user are displayed by
default. Much work was also done to streamline the code to make it
easier to follow. And to make it work correctly.
Use String#equals to match model name

Instead of String#matches, which is overly zealous in excluding matches.
Code maintenance

Clean up code; prune obsolete code; reorder packages.
Suppress the username label in Manage and Save menus

The username label for the current user must be associated with a model
when saving it. If the label isn't shown, it can't be deselected.