Help with F1
When user press <kbd> F 1 </kbd> a simple plain text help overlay dialog shows [the keyboard and mouse usage](https://github.com/dsrw/enu#keyboardmouse), 
this help in NOT toggle, if user releases the key then the help hides automatically, like a quick cheatsheet,
simple GUI, no animation, no complex design, just white text on a black screen is more than enough.
