doc: reorganize Doxygen groups/modules to make more sense

The Doxygen "Modules" page is not very illuminative, as different parts
of the API are mixed together and not all parts are included.

Try to address this:

Put all parts of the public API to some Doxygen group, usually one group
per header file. Use short, systematic names.

Make these groups sub-groups of a few top-level groups, roughly
corresponding to the different logical parts of the API (core, impl,
stream, filter, spa, utilities).

doc/Doxyfile.in

@@ -9,6 +9,7 @@ EXTRACT_ALL            = YES
 EXTRACT_STATIC         = YES
 FULL_PATH_NAMES        = YES
 STRIP_FROM_PATH        = @path_prefixes@
+STRIP_FROM_INC_PATH    = @path_prefixes@
 SHOW_INCLUDE_FILES     = NO
 GENERATE_TODOLIST      = NO
 GENERATE_TESTLIST      = NO