Fixed dead keys in X11.

The library will now try to create an X input context to handle dead
keys properly on international keyboards.  This makes it possible to
enter for example an e with accent grave on a German keyboard without
further efforts.  A fallback mechanism is provided in case the client
does not support X input method / context creation.  In that case, the
library will behave as it did before.

src/x11_platform.h

@@ -90,6 +90,8 @@ typedef struct _GLFWwindowX11
     // The last position the cursor was warped to by GLFW
     int             warpPosX, warpPosY;
 
+    // The window's input context
+    XIC ic;
 } _GLFWwindowX11;
 
 
@@ -202,6 +204,9 @@ typedef struct _GLFWlibraryX11
         Window      source;
     } xdnd;
 
+    // Input method and context
+    XIM im;
+
 } _GLFWlibraryX11;