Fix crash if magic_file() returns an error

Thanks to Benjamin Piwowarski who reported the problem and provided a fix.
magic_file() returns a NULL pointer if an error occurs, and due to a bug
this happens frequently on OS X: https://trac.macports.org/ticket/38771
I did not use the original patch, since I did not want to put a platform
specific workaround in (this needs further discussion), but the crash can
occur on all platforms and needs to be fixed.
avoid deployment of debug version of qt plugins
UserGuide.lyx: describe LyX's new extended font support
chkconfig.ltx: add package filehook

filehook.sty is required when you use non-TeX fonts and also the non-TeX math default font
In the same context LyX requires the font "latinmodern-math.otf", but we can currently not check for it, see my post to the list
Math.lyx: describe command \shortintertext
Fix crash after undo following replacement of a multicell selection (#8973).
Math.lyx: describe command \splitfrac
Math.lyx: describe mathtool's \xarrow commands
Math.lyx: describe \underbracket and \overbracket
Math.lyx: describe \smashoperator and how to insert ^ and _
Math.lyx: describe LaTeX-comment inset
Add FIXME
Fix bug #8884: Crash when "navigate to next change"

The two fixes here a obviously right, although it is not clear why they are sufficient to fix the bug. Anyway I cannot reproduce any crash with it.

 * the first part just conditions a whole if/else to change_next_pos.changed(). Originally, only the if branch was concerned.

 * the second part is to avoid calling CursorSlice::backwardPos() when position is 0. Doing this leads to an assertion.
Fix bug #8998: changed document setting is recoded twice as undo

The general trick to avoid this kind of problems is to use undo groups.
instant_preview.lyx: update .lyx format
instant_preview.lyx: load 'etex' package

The pgf package 3.0.0 update lead to an error being given with
this document. Loading the 'etex' package fixes the problem.
For more information, see:
  https://sourceforge.net/p/pgf/bugs/296/
Restore 'pdf' default format of Japanese docs

This restores 895f8285 and fixes out-of-the-box compilation.
Bug #9005: various things that do not work as intended in passthru paragraphs

* disable branch-add-insert in pass thru paragraphs

* when it is not possible to input a quote inset, insert a single
  ascii quote when argument to quote-insert is "single"

* handle "mathspace" dialog in Text::getStatus

* disable insertion of newline inset in pass thru paragraphs

* handle "mathdelimiter" and "mathmatrix" dialogs in GuiView::getStatus.