Add limit to the show/hide toggle rate.

When the keyboard repeat rate is set to a high value[1], holding
down the 'toggle guake visibility' button causes the screen to
blank out after 1~2 seconds.

While the ideal solution would be to implement some sort of locking
mechanism, I have a feeling that it would be a bit complicated..

[1]: Tested with 50 char/sec. On ubuntu, the slider is all the
     way to the right.