Adding @defgroup and @ingroup doxygen tags into all headers
in the common folder that contain doxygen blocks.
This improves the structure, readability, and findability
of information in the resulting output.
This commit targets purely structure and does not deal with
the content of the currently existing doxygen documentation.
Can be used with Callback<T> (means it's still type safe). It's used to
pass not only Request id to user's callback, but also a value user
wanted.
void *data field is removed from RequestInfo.
This commit also adds GlobalFunctionCallback, because it was needed in
order to replace plain C pointers to functions (which were used in
Request) into our object-oriented BaseCallback pointers.
These callbacks can call object's methods, not some global C functions.
DropboxStorage::info2() and DropboxStorage::infoMethodCallback()
demonstrate the idea.
