gtkplacessidebar: avoid unresponsive UI while trashing

gtkplacessidebar: avoid unresponsive UI while trashing

We were updating the whole places sidebar when the trash changed.
This effectively removes all rows and create new ones for every trash
state change.

Although when using GtkTreeView it was somehow ok, with the new
implementation with GtkListBox this effectively locks the UI while the
trash operations are being performed.

When performing operations for i.e. 100 files, the UI can be locked
for more than 1 minute since gvfs-trash usually takes time.

To fix this just update the icon of the trash when the state of the
trash change instead of the whole sidebar.

https://bugzilla.gnome.org/show_bug.cgi?id=762677