Add optional multitouch support to X11.

Thanks @TheBrokenRail for a patch proposal based on example code from esjeon (patch #322).
See https://sourceforge.net/p/irrlicht/patches/322
Original example code here: https://github.com/esjeon/xinput2-touch
Users have to enable _IRR_LINUX_X11_XINPUT2_ in IrrCompileConfig and link with Xi to make this work.
I rewrote the patch a bit and have no system for testing here, so this still needs some testing.
I also backported EET_TOUCH_INPUT_EVENT for this from the ogl-es branch.


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6178 dfc29bdd-3216-0410-991c-e03cc46cb475

source/Irrlicht/CIrrDeviceLinux.h

@@ -155,6 +155,8 @@ namespace irr
 
 		void initXAtoms();
 
+		void initXInput2();
+
 		bool switchToFullscreen(bool reset=false);
 
 #ifdef _IRR_COMPILE_WITH_X11_