Clipboard: code cleaning proposal

Here is not a bug but a proposal after a discussion on the developper mailinglist.

After a deep overview, I've found 4 functions in ClipboardManagerImpl which are not dedicated to (but are used by) the clipboard and could, sometimes, duplicate some already existing functions.

Finally I will keep one of those 4 function in place (_copyNode) because moving it in the Node class creates two much warnings for the moment.

I will post a patch wich will does following things :

Function void ClipboardManagerImpl::_pasteDocument(SPDesktop *desktop, SPDocument *clipdoc, bool in_place)
will become sp_import_document(SPDocument *clipdoc) in file.cpp
(TODO: look for a merge with file_import() in same file)

Function void ClipboardManagerImpl::_pasteDefs(SPDesktop *desktop, SPDocument *clipdoc)
will become SPDocument::importDefs(SPDocument *source)
(no existing function found)

Function SPCSSAttr *ClipboardManagerImpl::_parseColor(const Glib::ustring &text)
will become SPCSSAttr *sp_repr_css_attr_parse_color_to_fill() in xml/reps-css.cpp
(no existing function found)