Simplify getVisiblePages() and fix searching in presentation mode

This PR makes a couple of improvements of the presentation mode implementation:
* First of all, this patch removes the presentation mode specific code from `getVisiblePages()`, since it isn't actually needed any more. (It appears that this code was made redundant by https://github.com/mozilla/pdf.js/pull/2916, but I apparently missed it in that PR.)
This change is also necessary for the next part of this patch.

* Secondly, this patch also fixes issues related to searching in presentation mode. (Please note that searching is still *only* possible in the Firefox versions of PDF.js.)
~~This was done by adding a variable to the `PDFJS` object, since that was the easiest way I could think of that shouldn't break any third-party implementations of PDF.js with text selection enabled.~~

* Finally, since text selection isn't possible in presentation mode, the cursor is changed to properly reflect that.
Fixes #3423.