Linux/BSD: Modify only keypad keys

The `keycode` field of `InputEventKey` is supposed to be "unshifted";
That is, what the key would output if no modifier keys were pressed.
This should match what's written on the key label, but `Key` enumerates
also all keypad keys, which require a modifier. We thus require some
extra checks for them.

Note that this can still allow "stuck keys", but that's an even deeper
problem.

platform/linuxbsd/x11/key_mapping_x11.h

@@ -61,6 +61,7 @@ class KeyMappingX11 {
 public:
 	static void initialize();
 
+	static bool is_sym_numpad(KeySym p_keysym);
 	static Key get_keycode(KeySym p_keysym);
 	static unsigned int get_xlibcode(Key p_keysym);
 	static Key get_scancode(unsigned int p_code);