Implement an editor to customize the touchscreen controls (#14933)

- The editor is accessible via the pause menu and the settings menu.
- Buttons can be moved via drag & drop.
- Buttons can be added/removed. The grid menu added by #14918 is used to show
  all buttons not included in the layout.
- Custom layouts are responsive and adapt to changed screen size / DPI /
  hud_scaling.
- The layout is saved as JSON in the "touch_layout" setting.

src/client/game.cpp

@@ -26,6 +26,7 @@
 #include "client/event_manager.h"
 #include "fontengine.h"
 #include "gui/touchcontrols.h"
+#include "gui/touchscreeneditor.h"
 #include "itemdef.h"
 #include "log.h"
 #include "log_internal.h"
@@ -144,6 +145,11 @@ struct LocalFormspecHandler : public TextDest
 				return;
 			}
 
+			if (fields.find("btn_touchscreen_layout") != fields.end()) {
+				g_gamecallback->touchscreenLayout();
+				return;
+			}
+
 			if (fields.find("btn_exit_menu") != fields.end()) {
 				g_gamecallback->disconnect();
 				return;
@@ -1844,6 +1850,12 @@ inline bool Game::handleCallbacks()
 		g_gamecallback->keyconfig_requested = false;
 	}
 
+	if (g_gamecallback->touchscreenlayout_requested) {
+		(new GUITouchscreenLayout(guienv, guiroot, -1,
+				     &g_menumgr, texture_src))->drop();
+		g_gamecallback->touchscreenlayout_requested = false;
+	}
+
 	if (!g_gamecallback->show_open_url_dialog.empty()) {
 		(void)make_irr<GUIOpenURLMenu>(guienv, guiroot, -1,
 				 &g_menumgr, texture_src, g_gamecallback->show_open_url_dialog);
@@ -4510,9 +4522,14 @@ void Game::showPauseMenu()
 			<< strgettext("Sound Volume") << "]";
 	}
 #endif
-	os		<< "button_exit[4," << (ypos++) << ";3,0.5;btn_key_config;"
-		<< strgettext("Controls")  << "]";
 #endif
+	if (g_touchcontrols) {
+		os << "button_exit[4," << (ypos++) << ";3,0.5;btn_touchscreen_layout;"
+			<< strgettext("Touchscreen Layout")  << "]";
+	} else {
+		os << "button_exit[4," << (ypos++) << ";3,0.5;btn_key_config;"
+			<< strgettext("Controls")  << "]";
+	}
 	os		<< "button_exit[4," << (ypos++) << ";3,0.5;btn_exit_menu;"
 		<< strgettext("Exit to Menu") << "]";
 	os		<< "button_exit[4," << (ypos++) << ";3,0.5;btn_exit_os;"