Enable drag and drop to open files

Added the QT event handler to allow either a single file or multiple
files to be opened by drag and drop onto displaz window.

Tested on Ubuntu 12.04 64-bit.
Avoid crash when zero points read from text file

The crash was due to a floating point exception brought on by dividing
by zero.  Reading zero points and failing to get to EOF is now
considered a fatal text file format error.
Minor cleanup: use hasUrls() helper function
Minor cleanup of unused GL default format

This currently doesn't have any effect, so disable it for now.