Added SystemInfo dialog,

a new SLIC3R_BUILD define to be replaced by the build script,
a menu item to open the "New Issue" github page.

xs/xsp/XS.xsp

@@ -17,6 +17,12 @@ VERSION()
         RETVAL = newSVpv(SLIC3R_VERSION, 0);
     OUTPUT: RETVAL
 
+SV*
+BUILD()
+    CODE:
+        RETVAL = newSVpv(SLIC3R_BUILD, 0);
+    OUTPUT: RETVAL
+
 SV*
 DEBUG_OUT_PATH_PREFIX()
     CODE: