Minimize grid resizes

Don't resize the grid on _NET_WORKAREA changes when the grid size
doesn't actually change.
Update the wallpapers after user sets folder

If the user changes to a different folder with the gtk file chooser
button, it can no longer be set with gtk_file_chooser_set_current_folder.
Now, while the button won't get updated due to this bug in GTK, at
least the wallpapers will be current.