* messages.h:

* messages.h:
  - getMessages(), getGuiMessages(): new global function definitions

* lyx_main.C: implementation of getMessages() and getGuiMessages().

* LyX class:
  - getMessages(), getGuiMessages(), setGuiLanguage(): new interface for Messages access.

* LyX::Singletons class:
  - messages_: new container for Messages objects.

* Buffer::Impl::messages is now a pointer to the singleton defined in lyx_main.C.

* gettext.C: _() uses the global getGuiMessages() instead of the local static Messages object.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16673 a592a061-630c-0410-9148-cb99ea01b6c8