Experiment with replacing ModelMenu with a row of buttons

The row of buttons take up more space, but make it obvious what actions
can be performed. Plus, they can be selected with one click.
Add event handler for "open" ModelMenuPanel button

I'm breaking these out into separate classes, unlike all the inner
classes in ModelMenu.
Make static variables for frequently used FA icons

I put these in DataManager; should they go in Perspective? Or maybe a
new utility class in the "client.ui" package?
Employ excellent static variables for FA icons
Add event handler for "save" and "save as" buttons
Renamed ModelMenuPanelOpenHandler