Discern between virtual and abstract class bindings

* Previous "virtual" classes (which can't be instantiated) are not corretly named "abstract".
* Added a new "virtual" category for classes, they can't be instantiated from the editor, but can be inherited from script and extensions.
* Converted a large amount of classes from "abstract" to "virtual" where it makes sense.

Most classes that make sense have been converted. Missing:

* Physics servers
* VideoStream
* Script* classes.

which will go in a separate PR due to the complexity involved.

modules/minimp3/audio_stream_mp3.cpp

@@ -83,7 +83,7 @@ void AudioStreamPlaybackMP3::start(float p_from_pos) {
 	active = true;
 	seek(p_from_pos);
 	loops = 0;
-	_begin_resample();
+	begin_resample();
 }
 
 void AudioStreamPlaybackMP3::stop() {