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.

doc/classes/Resource.xml

@@ -12,6 +12,11 @@
 		<link title="When and how to avoid using nodes for everything">$DOCS_URL/tutorials/best_practices/node_alternatives.html</link>
 	</tutorials>
 	<methods>
+		<method name="_get_rid" qualifiers="virtual">
+			<return type="RID" />
+			<description>
+			</description>
+		</method>
 		<method name="duplicate" qualifiers="const">
 			<return type="Resource" />
 			<argument index="0" name="subresources" type="bool" default="false" />