[X11] Reconcile logical keyboard state with physical state on FocusIn

since the window system doesn't do it for us like other platforms.

This prevents sticky keys and missed keys when going in and out
of focus, for example Alt would appear to stick if switching away
from an SDL app with Alt-Tab and had to be pressed again.

CR: Sam

visualtest/include/SDL_visualtest_mischelper.h

@@ -0,0 +1,26 @@
+/**
+ * \file mischelper.c 
+ *
+ * Header with miscellaneous helper functions.
+ */
+
+#ifndef _SDL_visualtest_mischelper_h
+#define _SDL_visualtest_mischelper_h
+
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Stores a 32 digit hexadecimal string representing the MD5 hash of the
+ * string \c str in \c hash.
+ */
+void SDLVisualTest_HashString(char* str, char hash[33]);
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _SDL_visualtest_mischelper_h */