Detach Buffer preview and update preview in a new thread. Only for Qt4.4 users. This...

Detach Buffer preview and update preview in a new thread. Only for Qt4.4 users. This new feature can be disabled by setting EXPORT_in_THREAD to 0 in GuiView.cpp.
Two things are missing (but are fixable):
- the lack of feedback of the background latex compilation
- the error list is not shown in case of compilation error.

* Buffer:
- create a "cloned buffer" type.
- Transfer LFUN_MASTER_BUFFER_UPDATE, LFUN_MASTER_BUFFER_VIEW, LFUN_BUFFER_UPDATE and LFUN_BUFFER_VIEW to GuiView. This is good itself as these LFUN are GUI oriented.

* GuiView: detach the above LFUNs to a new thread as was already done for autosave.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@32584 a592a061-630c-0410-9148-cb99ea01b6c8