Add interval sort function

.. added to the splitter handle context menu

.. also took IntervalItem.h and IntervalTreeView.h from MainWindow.h
   coz I was getting pissed with recompiling everything when changing it.
   Thats why so many other files have been changed in this commit.

Fixes #338.
Display negative temperatures (and respect temperature units) in AllPlot
fixes #466
Missed ModelPlot for IntervalItem.h

.. after removing it from MainWindow.h
Correct min value for speed axis
Remove rename single interval functionality

.. since it is superceded by the ability to edit all
   the interval details, including start and stop time
Documentation: User Documentation Content

.. restructured the doc directory to distibguish between
   website content and user documentation

.. added basic content for FAQ and Manual, but neither
   are complete and contain no markup
Documentation: Completed FAQ content

.. all questions answered.

.. pulled in R.Chung's paper on VE to reference in
   one of the answers. It gets stored for posterity now.
Documentation: Initial FAQ

.. completed and fixed up content
.. initial basic texinfo markup done
Documentation: Fix menu for makeinfo on FAQ

.. the nodes needed next, prev, up set so we could
   create an info file using makeinfo
Documentation: Added a Makefile

.. to create pdf, html and info files
.. and .gitignore to ignore outputs
Documentation: Filenames too generic

.. So FAQ becomes GC3-FAQ
.. and Manual becomes GC3-Manual

This is just so they can be distributed and installed in their own
right and still be identifiable as GoldenCheetah v3 files.
Documentation: Add the doc outputs to git

.. no longer ignored in .gitignore

.. they are committed to the repo in the same
   way that translation .qm files are

.. in this way those without the right tools installed
   (in this case tex and texinfo tools) can still use
   them.
UI tweaks: Slightly darker toolbar buttons

.. on Linux and Windows to match the toolbar background.