Fixed mnemonic selection

By default the list box control selects any item with the same starting
letter as any key pressed while the list has focus. This means the user
can accidentally select combatants in the initiative order by entering
shortcuts like Ctrl + W (would select first combatant with "W" as the
first letter) or Ctrl + D (would select "Dave") etc etc.
Tweaked setCurrent and setMax buttons

"Set Current" now stops user from assigning a new current HP higher than
maximum. Also, when the new maximum is set it increases the current HP
by the same amount the maximum was increased by.