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/gui/CMakeLists.txt

@@ -25,5 +25,7 @@ set(gui_SRCS
 	${CMAKE_CURRENT_SOURCE_DIR}/modalMenu.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/profilergraph.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/touchcontrols.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/touchscreenlayout.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/touchscreeneditor.cpp
 	PARENT_SCOPE
 )