Replace inline favorite action with just a favorite icon

This is a preparatory step for a following commit in which the position
of the favorite icon and the checkbox will be swapped; in that new
design the favorite icon is no longer expected to be an action but just
a simple mark on whether the file is favorited or not (the action is
expected to be triggered then only from the file actions menu).

The favorite icon is now fully shown or completely hidden depending on
whether the file is favorited or not. As the icon is just informative
but no longer an action now it does not change when hovered or focus. In
the same way, the alternative text when the file is not favorited now it
is not "Favorite" (an action) but "Not favorited" instead.

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