Add scroll_container formspec element (redo) (#9101)

New formspec elements:

 - `scroll_container[<X>,<Y>;<W>,<H>;<scrollbar name>;<orientation>;<scroll factor>]`
 - `scroll_container_end[]`

Other elements can be embedded in this element. Scrollbar must be placed manually.

src/gui/guiScrollContainer.h

@@ -0,0 +1,56 @@
+/*
+Minetest
+Copyright (C) 2020 DS
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+
+#include "irrlichttypes_extrabloated.h"
+#include "util/string.h"
+#include "guiScrollBar.h"
+
+class GUIScrollContainer : public gui::IGUIElement
+{
+public:
+	GUIScrollContainer(gui::IGUIEnvironment *env, gui::IGUIElement *parent, s32 id,
+			const core::rect<s32> &rectangle, const std::string &orientation,
+			f32 scrollfactor);
+
+	virtual bool OnEvent(const SEvent &event) override;
+
+	inline void onScrollEvent(gui::IGUIElement *caller)
+	{
+		if (caller == m_scrollbar)
+			updateScrolling();
+	}
+
+	inline void setScrollBar(GUIScrollBar *scrollbar) { m_scrollbar = scrollbar; }
+
+private:
+	enum OrientationEnum
+	{
+		VERTICAL,
+		HORIZONTAL,
+		UNDEFINED
+	};
+
+	GUIScrollBar *m_scrollbar;
+	OrientationEnum m_orientation;
+	f32 m_scrollfactor;
+
+	void updateScrolling();
+};