First step of InsetLayout cleanup.

First step of InsetLayout cleanup.

Get rid of Inset::getLayout(BufferParams), since all insets have access to
their own buffer. Inset::getLayout() does the same now.

Get rid of InsetCollapsable::getLayout(BufferParams) too, and remove most references to InsetCollapsable::layout_. For now IC::getLayout() still returns layout_, but the idea is to change this (provided there is no performance problem).

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30530 a592a061-630c-0410-9148-cb99ea01b6c8