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/hud.cpp

@@ -24,6 +24,7 @@
 #include "wieldmesh.h"
 #include "client/renderingengine.h"
 #include "client/minimap.h"
+#include "client/texturesource.h"
 #include "gui/touchcontrols.h"
 #include "util/enriched_string.h"
 #include "irrlicht_changes/CGUITTFont.h"