This commit adds a generic mouse-timeout plugin. (Same code was
originally part of toolbar plugin, but is now general purpose and
available to user to apply any CSS to it.)
Although this implementation is different and more generic, the
suggestion to add ability to hide mouse cursor came from
a pull request by Sebastian Clausen (@sclausen):
impress#536
The functionality is simple:
After 3 seconds of mouse inactivity, add the css class
`body.impress-mouse-timeout`. On `mousemove`, `click` or `touch`, remove the
class.
A user will then use (or not) his own CSS to hide whatever he wants to hide
after 3 seconds of mouse inactivity.
