* GuiView::closeEvent(): adjust buffer closing WRT multi-view and child documents.

* GuiView::closeEvent(): adjust buffer closing WRT multi-view and child documents.
- a child document is kept loaded in any case. It is saved if needed before being hidden.
- a document which is not viewed in another window is closed now.
- we do nothing for documents that are also viewed in another window. There is two FIXME related to this last item in the code:

// FIXME 1: should we put an alert box here that the buffer
// is viewed elsewhere?
// FIXME 2: should we try to save this buffer in any case?
//saveBuffer(b);

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23250 a592a061-630c-0410-9148-cb99ea01b6c8