This commits cuts down LyX::exec2 into two new methods (execBatchCommands() and resto...

This commits cuts down LyX::exec2 into two new methods (execBatchCommands() and restoreGuiSession()) and introduce a number of helper private function to sanitize the initialization and exit process; quitLyx() is replaced with LyX::quit(). The Application starting and reset is done directly in priv_exec().

* lyx_cb.[Ch]: delete quitLyX()

* lyx_main.[Ch]
  - lyx_exit(): moved to LyX::earlyExit()
  - prepareExit(), quit(), earlyExit(), execBatchCommands(), initGuiFont(): new LyX methods.
  - LyX::priv_exec(): restructured with above helper methods.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15353 a592a061-630c-0410-9148-cb99ea01b6c8