keyboard: Add some SDL keycodes for common Xkb keys

Add SDL keycodes for keys found commonly found in the default Xkb layout, such as left tab and compose, and keys frequently used for custom modifiers such as Meta, Hyper, and Level5 Shift.

As these keys aren't Unicode code points and don't have associated scancodes (at least on modern keyboards), they are placed in the new extended key code space, with bit 30 set as a flag.

test/checkkeys.c

@@ -167,6 +167,9 @@ static void print_modifiers(char **text, size_t *maxlen, SDL_Keymod mod)
     if (mod & SDL_KMOD_MODE) {
         print_string(text, maxlen, " MODE");
     }
+    if (mod & SDL_KMOD_LEVEL5) {
+        print_string(text, maxlen, " LEVEL5");
+    }
     if (mod & SDL_KMOD_SCROLL) {
         print_string(text, maxlen, " SCROLL");
     }