installer: fix an issue

- also take care of emergency releases when updating LyX
- add some prerequisites for a new feature
The update here needs doing no matter what.

This fixes a bug introduced in db358a4315.
Minor cleanup.
header_check.sh: allow exclusion of common headers

Continuing to address #6305
Don't consider unnumbered sections for tocdepth.
UserGuide.lyx: next revision step

- revised sec. 6 - 6.6
- describe longest label feature for bibliography environment
Comment.
ViewSourceUi.ui layout was broken
Set "dryrun" to true for XHTML copying routine. This suppresses
the production of images, etc, making copying even the entire
User's Guide and almost instantaneous operation.
Add layout intelligence to View Source pane (bug #8653).

View Source pane layout now automatically switches between horizontal
and vertical design when docked in vertical or horizontal way.

Original idea and patch from Edwin.

http://www.mail-archive.com/lyx-devel@lists.lyx.org/msg178801.html

Note: the long standing-fight with too bulky minimal QTextEdit size
is now solved via ViewSourceUi.sizePolicy=Ignored.
Automatic horiz/vert switch of layout when docking Advanced F&R pane top-bottom/left-right (copying 49c2450f).
Automatic horiz/vert switch also for Advanced Tab (completing e3677975).
GuiBox.cpp: fix a dialog initialization issue

thanks Jean-Marc for the hint
tex2lyx/text.cpp: fix a typo spotted by clang
Revert "tex2lyx/text.cpp: fix a typo spotted by clang"

This reverts commit 055b425b646e748e0c35f0eef89e03e6126a143d.

This requires much more work and i don't have time to do this now
Fix bug 6055: change-next does not work in tables

The problem was just the faulty use of CursorSlice::at_begin/end(), which does not look for end of cell, but end of inset.
Small clanup (just move functions around)
Another try at making the source view dock arrange itself.

The idea is to minimize the size of the form that contains the buttons.

Since I do not really know what I am doing, I will revert if problems occur.
Remove cruft