gtkplacesview: rotate server list icon on toggled

gtkplacesview: rotate server list icon on toggled

Disclosure triangles are usually used pointing down, however
in this case the popover spawns in the upper direction, which
makes it odd looking.
Instead of pointing always down or up, point down when not toggled and
animate a rotation when toggled.

https://bugzilla.gnome.org/show_bug.cgi?id=756568