X11: Query and keep track of Xkb group index

For users with multiple keyboard layouts configured, glfwGetKeyName
works fine only with the primary layout.  Switching layouts results in
changing the group index.  This commit querries the current group index
when initializing keyboard input and keeps track of any change to it.

As a result the scancode -> keyname mapping may change while the program
is running (needs to be documented).

Fixes #1462.
Closes #1528.

src/x11_platform.h

@@ -323,13 +323,14 @@ typedef struct _GLFWlibraryX11
     } randr;
 
     struct {
-        GLFWbool    available;
-        GLFWbool    detectable;
-        int         majorOpcode;
-        int         eventBase;
-        int         errorBase;
-        int         major;
-        int         minor;
+        GLFWbool     available;
+        GLFWbool     detectable;
+        int          majorOpcode;
+        int          eventBase;
+        int          errorBase;
+        int          major;
+        int          minor;
+        unsigned int group;
     } xkb;
 
     struct {