This makes it feasible to merge the LV2 documentation into more broad
documentation in a meta-project.

Similar to the previous commit, this was non-standard and caused documentation
issues.  With this commit, the data structures index page looks as expected.

Unlike the previous commit, though, these names are actually needed because the
struct definition refers to itself.  Simply use the same name instead of the
underscore prefix, since the "struct namespace" in C is different anyway.

These are not necessary since they are not recursive types, and the names
violated the C standard.  There also seems to be a bug in Doxygen where
TYPEDEF_HIDES_STRUCT was not working, which made the documentation confusing
and ugly.

Strictly speaking, this could break user code that used these names directly,
but this was never intended to be supported and should be fixed if it actually
occurs anywhere.