Add a pref for animating images on hover

Closes #5.
Use image.animation_mode for managing default animation mode

Closes #2.
Fix position of hover buttons inside tables

Closes #8.
Ctrl+M should keep the individually toggled image

It makes no difference in the usual case, but there are two potential benefits:
- When you open an image in another tab and it starts animating, and you
  switch back to the original tab (where the image then animates because
  animation state of the same image is shared), you want the opened image
  to be the individually toggle image. This can help it become that.
- With "animate on hover" enabled, this fixes the case of hovering the
  image, pressing Ctrl+M, and hovering it again.
Default "play on hover" to off.
Fix indentation
1.1