fix useless update checks
change event listener for button command
remove unused key |name| of preset item
remove inefficient constant to wrap |button.type|
fix unnecessary |type| attribute of a normal button
define meaningful names for preference values
add image animation switch button
minor fix

* update comment
* |var| -> |let|
implement Shift+ESC for stopping image animation

* ESC key function has been changed from Fx20
  see https://bugzilla.mozilla.org/show_bug.cgi?id=614304