db/Interface: CollectUniqueTags() allows multiple "groups"

Instead of passing tag and group, pass an array of tags.  To support a
nested return value, return a nested std::map of std::maps.  Each key
specifies the tag value, and each value may be another nesting level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/408

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -42,6 +42,8 @@
 #include "fs/FileSystem.hxx"
 #include "util/CharUtil.hxx"
 #include "util/Domain.hxx"
+#include "util/ConstBuffer.hxx"
+#include "util/RecursiveMap.hxx"
 #include "Log.hxx"
 
 #ifdef ENABLE_ZLIB
@@ -329,11 +331,11 @@ SimpleDatabase::Visit(const DatabaseSelection &selection,
 			    "No such directory");
 }
 
-std::map<std::string, std::set<std::string>>
+RecursiveMap<std::string>
 SimpleDatabase::CollectUniqueTags(const DatabaseSelection &selection,
-				  TagType tag_type, TagType group) const
+				  ConstBuffer<TagType> tag_types) const
 {
-	return ::CollectUniqueTags(*this, selection, tag_type, group);
+	return ::CollectUniqueTags(*this, selection, tag_types);
 }
 
 DatabaseStats