[RFC] Enum improvements

[Yuriy Skalko]

Now LyX requires C++11 and we can use its new enum features:

1. Enum base type. It will allow forward declarations of enums in 
headers to break weak, unnecessary dependencies like BufferParams.h ---> 
InsetQuotes.h. This will also improve compilation speed.

2. Scoped enums. It will allow better type safety. Also some commonly 
used enum declarations can be moved out of classes (that used solely for 
limiting scope of enum values).


Yuriy