Fixes some solution number woes

[Jimvy]

• Adds a solution* environment. This one doesn't display the number of the question/exercise, so it doesn't display the words "à la Question" or "à l'Exercice". The label is thus quite short, maybe too short.
• By default, the solution now uses \thetitle, provided by titlesec, as label. This adapts to sections, subsections, even paragraphs. Of course, starred sections (that don't change the counter) have still a problem with it, but this is only marginally problematic.