[kcms/keyboard] tastenbrett: fix diacritics display in keyboard preview
In the keyboard preview, most diacritics from the internal QHash<unsigned long, char> deadMap don't show up, since their values don't fit on a char.
On my system, non-spacing diacritics still don't show up after fixing that. Unicode recommends applying them to NBSP to exhibit them in isolation. However, doing that, I get the diacritic applied to what appears to be the replacement character (the enclosed interrogation thingie). So apply them to a SPACE character (not recommended since Unicode 4.1).
While we're at it, change the handakuten from its non-spacing mark to its spacing symbol variant for good measure.
Also "_" doesn't appear, since it is turned to a blank by the logic to turn "Control_L" to "Control L".