Improve input event accumulation

- API has been simplified: all events now go through `parse_input_event()`. Whether they are accumulated or not depends on the `use_accumulated_input` flag.
- Event accumulation is now thread-safe (it was not needed so far, but it prepares the ground for the following changes).
- Touch drag events now support accumulation.

core/input/input_event.h

@@ -410,6 +410,8 @@ public:
 	virtual String as_text() const override;
 	virtual String to_string() override;
 
+	virtual bool accumulate(const Ref<InputEvent> &p_event) override;
+
 	InputEventScreenDrag() {}
 };