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.

servers/audio/audio_effect.cpp

@@ -30,5 +30,36 @@
 
 #include "audio_effect.h"
 
+void AudioEffectInstance::process(const AudioFrame *p_src_frames, AudioFrame *p_dst_frames, int p_frame_count) {
+	if (GDVIRTUAL_REQUIRED_CALL(_process, p_src_frames, p_dst_frames, p_frame_count)) {
+		return;
+	}
+}
+bool AudioEffectInstance::process_silence() const {
+	bool ret;
+	if (GDVIRTUAL_CALL(_process_silence, ret)) {
+		return ret;
+	}
+	return false;
+}
+
+void AudioEffectInstance::_bind_methods() {
+	GDVIRTUAL_BIND(_process, "src_buffer", "dst_buffer", "frame_count");
+	GDVIRTUAL_BIND(_process_silence);
+}
+
+////
+
+Ref<AudioEffectInstance> AudioEffect::instantiate() {
+	Ref<AudioEffectInstance> ret;
+	if (GDVIRTUAL_REQUIRED_CALL(_instantiate, ret)) {
+		return ret;
+	}
+	return Ref<AudioEffectInstance>();
+}
+void AudioEffect::_bind_methods() {
+	GDVIRTUAL_BIND(_instantiate);
+}
+
 AudioEffect::AudioEffect() {
 }