* src/LaTeXFeatures.C (mustProvide): new method; returns true when
isRequired is true and the feature is not provided by the
textclass.
(getPackages, getLyXSGMLEntities): use mustProvide when relevant.
* src/bufferparams.C (getEngine):
* src/buffer.C (validate): adapt to change below.
* src/lyxtextclass.C (read): replace tags ProvidesNatbib,
ProvidesMakeidx, ProvidesUrl and ProvidesAmsmath by a generic
"Provides".
(provides): take a string as argument.
* lib/scripts/layout2layout.py (convert): convert "ProvidesXxx" tags
to "Provides xxx".
* lib/layouts/*: update to format 4.
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17740 a592a061-630c-0410-9148-cb99ea01b6c8
