Widget drawing code already supported drawing right-aligned, grayed out shortcut strings. This patch generalizes things a bit so this can also be used to draw other hints in the same way. There have been a few instances in the past where this would've been useful, D11046 being the latest one. Note that besides some manual regression testing, I didn't check if this works yet, as there is no code actually using it (other than the shortcuts). Can be checked as part of further development for D11046. A possible further improvement would be providing a way to define how clipping should be done. E.g. sometimes the right-aligned text should be clipped first (because it's just a hint), in other cases it should be left untouched (like current code explicitly does it for shortcuts). Removes the `UI_BUT_HAS_SHORTCUT` flag, which isn't needed anymore.