Added image precache support (main viewport) - refs #24

The main viewport and template now support "preloading" certain images
within a hidden div so UI elements are always already loaded. Hard coded
a set of basic images (tabs, borders, etc) as well as warning and error
icons so they will display correctly in the event of a server failure
(since if the server is down, it is already too late to fetch the error
icon to display with the message saying that the server is down)

Also removed unused/old 'AppMaster' class