Partial fix to bug 2355.

Partial fix to bug 2355.

	* src/bufferparams.C
	(bool BufferParams::writeLaTeX):
	(void BufferParams::useClassDefaults): only take action when there is
	a ToC structure

	* src/frontends/qt2/QDocument.C
	(void QDocument::apply):
	(void QDocument::update_contents): only take action when there is
        a ToC structure

	* src/frontends/xforms/FormDocument.C
	(bool FormDocument::language_apply):
	(bool FormDocument::options_apply):
	(void FormDocument::options_update): only take action when there is
        a ToC structure; disable
	(void FormDocument::UpdateClassParams): disable the secnumdepth and
	tocdepth widgets when there is no ToC structure

	* src/frontends/xforms/FormDocument.h
	(options_apply): do not return a bool (was not used)

	* src/lyxtextclass.C
	(hasTocLevels): new method.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13471 a592a061-630c-0410-9148-cb99ea01b6c8