This commit transfers the singletons defined in the Application class to the private...

This commit transfers the singletons defined in the Application class to the private LyX::Singletons implementation. It also put the session_ member and top_level_keymap global variable into this Singleton structure.
This commit also modifies the LyXFunc class. A LyXView is not needed any more at construction. Neither is the top level keymap. The LyXFunc key sequences are initialized only if we are in GUI mode (lyx::use_gui = true).
The next step is to rename LyXFunc::owner to LyXFunc::lyx_view_ and to put some BOOST_ASSERT. A classification between Gui and Kernel oriented LFUNs would also be welcome.

*LyX/lyx_main.[Ch]:
 - LyX::Singletons: new private structure containing all the singletons (except the main LyX one of course).
 - buffer_list_, top_level_keymap, session_: transfered to the LyX::Singletons structure.

* LyXFunc:
  - LyXFunc: default constructor does not need any arguments.
  - setupLocalKeymap(): deleted (was not used anywhere).
  - initKeySequences(): new public method called from lyx_main.C, useful only in lyx::use_gui mode.
  - new private member accessor methods

* Application:
  - Application_pimpl: deleted (transfered to LyX)
  - global singleton accessors: transfered to lyx_main.C
  - start(): code transfered to LyX::priv_exec()

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15367 a592a061-630c-0410-9148-cb99ea01b6c8