Introduce a new "RequiredArgs" tag for layouts. This functions much as

Introduce a new "RequiredArgs" tag for layouts. This functions much as
the OptionalArgs tag does and is implemented by the now misnamed
InsetOptArgs, except that its content gets wrapped in "{}" rather than
"[]". Required arguments do not actually have to be provided, but they
are always output.

This will allow e.g. beamer's Block environment to be implemented
without ERT.

Documentation to follow.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@34591 a592a061-630c-0410-9148-cb99ea01b6c8