Fix GuessWhat to prevent multiword hint exploit

After the change to multiword parsing (commit 7453f3b9), it became much
easier for users to enter multiword hints. This has been fixed using
the same code that ZenTag uses, with some slight modifications. This
limits the characters that can be entered in the text area to just
letters (a-z, A-Z).
Add copy/cut/paste prevention to the word field in all games

Although most games filter out various forbidden characters, this is
done using the "keydown" event, which means that characters entered
by pasting are not filtered. To solve this problem, a function has been
added to MG_GAME_API (preventEvents()) which can be used to prevent
pasting, copying, etc. In addition, calls to this function have been
added to each individual game.
Merge branch 'gsolbeck-development' into development: GuessWhat prevent multiword hint exploit