Added base methods to ResourceViewer.

These methods must be overridden by implementing classes:
load()
resize()
Buttons in the ResourceViewer top panel are now conjoined
Top panel takes up minimal space, and is translucent before mouseover
Adapted ErrorScreen from resource-audio branch
ResourceViewers can now display an error message

Simply use showError([message]) and hideError().
Adapted ProgressBar from resource-audio branch
ResourceViewers can now display and hide animated progress bars

Use the following methods to manipulate the progress bar:

- showProgressBar([initialLoadingMessage])
- updateProgressBar(percent, [loadingMessage])
- hideProgressBar()

If initialLoadingMessage is not provided, a default message will be displayed instead.
Tweaked progress bar to display faster transitions on percentage updates