Trigger events before and after a file action is executed

In the same way that other elements can know when a FileAction is
registered or a default action is set this commit makes possible to be
notified before and after a FileAction is executed.

This is achieved by wrapping the registered action handler in a custom
function that notifies the listeners before and after executing the
handler itself. Due to this approach only FileActions registered through
"registerAction" trigger the events, although that is not a problem as
this is how the actions should be added to the FileActions anyway.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>