Implemented controller-view split for FormError and FormInclude.

Implemented controller-view split for FormError and FormInclude.
Cleaned up code in controllers in the process; I think that the interface
is stabilising now.
Cleaned up code in InsetInclude a little.

Mental note (to me?!): the Bibitem, Bibtex and Include insets create an
inset when the appropriate menu item is selected. This is incorrect;
the inset should only be created when "Ok" is pressed, as with the
majority of insets. CREATE_XXX flags and associated code are needed.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@1817 a592a061-630c-0410-9148-cb99ea01b6c8