Add mozPrintCallback shim

This shim does the following:
1. Intercept window.print()
2. For a window.print() call (if allowed, ie. no previous print job):
   1. Dispatch the beforeprint event.
   2. Render a printg progress dialog.
   3. For each canvas, call mozPrintCallback if existent (one at a time, async).
   4. During each mozPrintCallback callback, update the progress dialog.
   5. When all <canvas>es have been rendered, invoke the real window.print().
   6. Dispatch the afterprint event.
Resolved severe memory leak (mozPrintCallback)

Do NOT save the temporary <canvas> element as `this.canvas`.
`PDFViewer.pages[i].canvas` appears to be used to generate the thumbnail
icons. Well, that's no justification for preventing GC of those
temporary <canvas> elements used during mozPrintCallback.

With a document consisting of 79 pages, this resulted in a 600-700MB
leaked memory.
Wrap printed <canvas> in <div> for Chrome

Otherwise one page (canvas) will consume two (printed) pages in Chrome.
This method has no side-effects in Firefox.
Keyboard shortcuts - intercept Ctrl/Cmd + P / Esc