Replace PDFReader with PDFDebugger

As discussed on the mailing list:

Here's an idea: if we switch PDFDebugger to using "View Pages" by default, it will no longer be confusing for casual users. I've found myself using this mode most of the time anyway. We can add page up/down too, of course - preferably using the actual "Page Up" and "Page Down" keys rather than the bizarre choice of the +/- keys which are currently used in PDFReader.