*** Requires tag for classes, layouts and custom insets ***

*** Requires tag for classes, layouts and custom insets ***

* src/TextClass{cpp,h}:
	- implement Requires tag for class and InsetLayout.
* src/Layout.{cpp,h}:
	- implement Requires tag for layouts.
	- increase layouts format to 6.
* src/Paragraph.cpp:
	- request required features for paragraph layouts.
* src/LaTeXFeatures.cpp:
	- add some packages needed by modules.
	- remove Preamble method for InsetLayout. This is now done
	  in InsetFlex::validate.
* src/BufferParams.cpp (validate):
	- request required features for the class.
* src/insets/InsetLayout.h:
	- new member "requires".
* src/insets/InsetFlex.{cpp,h}:
	- implement validate method and request required features and preamble snippets.

* lib/layouts/*.{layout,module,inc}:
	- increase layouts format to 6.
	- use Requires tags in some modules (more can be done later).

* lib/doc/Customization.lyx:
	- document Requires tag.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@22414 a592a061-630c-0410-9148-cb99ea01b6c8