Make "devel mode" configurable at run time

Make "devel mode" configurable at run time

Traditionally LyX behaves differently when the directive DEVEL_VERSION
is defined at compile time. This covers
* more detailed description of current position in status bar
* the help files are open in read/write mode
* more detailed debug output in the View Source panel

This patch introduces the new function devel-mode-toggle that allows
to use devel mode in stable releases, and vice versa.

The information is saved in the session file. The default is to
disable devel mode.

Remove all traces of DEVEL_VERSION in autoconf and cmake