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.