Added typeface initialization to CustomLookAndFeel, and implemented
getTypefaceForFont().

Fonts can now be created using a more centralized method, by calling the
Font(String("Typeface Name"), fontHeight, Font::style) constructor.  Names for
the typefaces ("Default Light", "Paragraph") should be regarded as tentative.

There's also simpler code to do the same thing that should be uncommented after
the 2.0 update (it uses HashMap, which is not currently in our library code,
even though it's in the 1.53 docs.)

Before, EditorViewport would start accessing arrays on keypresses (specifically
the arrow keys) even when there were no editors.  This adds an extra check to
prevent this.

Sometimes initialize() returns no error string, but will not find an audio
device, leaving aIOd == null.  This checks for that, and aborts the app with an
informational dialog window if so.

If there was a problem initializing the AudioDeviceManager, we show an
AlertWindow with Retry/Quit options.  Unfortunately, this doesn't fix the bug I
was initially trying to fix, because sometimes initialize() returns OK while
setting the AudioDeviceManager to null.