Refactor Buffer.cpp: loadLyXFile():

Refactor Buffer.cpp: loadLyXFile():

- make a new function readFromVC(),
if the file does not exist, try to extract it from the version control (RCS),

- make a new function readEmergency(),
try to read the emergency file that is associated to the file we are trying to read,

- make a new function readAutosave(),
try to read the autosave file that is associated to the file we are trying to read,

- merge loadLyXFile() and readFileHelper(),
this new function now determines which file to read.

- add more ReadStatus elements to describe failures.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@35819 a592a061-630c-0410-9148-cb99ea01b6c8