Convert all letters to upper case to compare.

BUG: 362193
Since special characters for each language in khangman language.txt
files are lower case, but we show upper case in the gui special
characters were not properly getting marked as disabled or being
marked green or red when clicked. To fix always compare
upper case version of letters.
Note: if I changed letter button letter property to modelData.toUpperCase
some German characters were shown incorrectly in the gui, so show the
modelData with capitalization font, but compare toUpperCase() behind
the scenes.