* TocBackend.[Ch]: new files defining class TocBackend. This class contains adapted...

* TocBackend.[Ch]: new files defining class TocBackend. This class contains adapted code from former toc.C

* toc.[Ch]:
    - rewritten to take advantage of new TocBackend class. The toc
    cache is implemented as a static variable:
    static map<Buffer const *, lyx::TocBackend> toc_backend_;

* buffer_funcs.C:
    updateLabels(Buffer const & buf) now calls
    "lyx::toc::updateToc(buf);"

* pariterator.h: added default constructor
    ParConstIterator(): DocIterator() {}

* insetfloat.C: added pit parameter to TocItem construction

* insetwrap.C: added pit parameter to TocItem construction

* MenuBackend.C: use a const ref instead of a copy of TocList

* ControlToc.[Ch]: optimisation of the API by using const reference instead of copy.

* qt4/TocPanel.[Ch]:
    - optimisation of the API by using const reference instead of
    copy
    - directly use of TocBackend::TocIterator instead of identification by paragraph contents.

* qt4/QToc.C:
    - optimisation of the API by using const reference instead of
    copy
    - makes use of TocBackend::Item::uid()

* qt2/QToc.C:
    - use TocItem::depth() and TocItem::str() instead of public member access.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13713 a592a061-630c-0410-9148-cb99ea01b6c8