Finally set a window icon on X11

Since the creation of minetest, it had no window icon on X11.
Now we have one.

The misc/minetest-xorg-icon-128.png file is a rendering of the
misc/minetest.svg file with inkscape, created with something like:

inkscape -z -e misc/minetest-xorg-icon-128.png -w 128 -h 128 misc/minetest.svg

src/porting.h

@@ -367,6 +367,9 @@ inline const char *getPlatformName()
 void setXorgClassHint(const video::SExposedVideoData &video_data,
 	const std::string &name);
 
+bool setXorgWindowIcon(IrrlichtDevice *device,
+	const std::string &icon_file);
+
 // This only needs to be called at the start of execution, since all future
 // threads in the process inherit this exception handler
 void setWin32ExceptionHandler();