Fix double-rendering of text in "Solve" button of the problem navigation context.

Fix the text of "Solve: ..." button/menu in the problem navigation
context being rendered two times and overlaid.

Don't know whether not giving the text to style options will have any
nasty side-effects, but well... Problem navigation context is so far the
only user of the rich-text push buttons.

REVIEW: 108144
CCBUG: 302439