Refactor Process Mode

Implements https://github.com/godotengine/godot-proposals/issues/1835#issuecomment-727186192

* PauseMode is now ProcessMode, containing the following states:
	```
	PROCESS_MODE_INHERIT, // same as parent node
	PROCESS_MODE_NORMAL, // process only if not paused
	PROCESS_MODE_PAUSE_ONLY, // process only if paused
	PROCESS_MODE_ALWAYS, // process always
	PROCESS_MODE_DISABLED, // never process
	```
* NOTIFICATION_PAUSED and NOTIFICATION_UNPAUSED are received effectively when the node is paused and unpaused (not any longer when pause mode is set in SceneTree).
* Renamed some nodes that used ProcessMode/process_mode to specify a callback type to ProcessCallback to avoid clashes.

editor/editor_inspector.cpp

@@ -1761,6 +1761,10 @@ void EditorInspector::update_tree() {
 			continue;
 		}
 
+		if (p.name == "script") {
+			category_vbox = nullptr; // script should go into its own category
+		}
+
 		if (p.usage & PROPERTY_USAGE_HIGH_END_GFX && RS::get_singleton()->is_low_end()) {
 			continue; //do not show this property in low end gfx
 		}