keybindings: Rebrand "minimize" as "hide"

The minimize action does not make much sense without a window list,
which is why its default keybinding was removed in bug 643609;
getting the focused window out of the way quickly can still be useful
though, so change the description to "Hide window" instead, which does
not imply a window list. We will then assign a default shortcut again.

https://bugzilla.gnome.org/show_bug.cgi?id=682887