Make fd.o ShowItemProperties behavior consistent with Thunar

Previously the D-Bus interface method
org.freedesktop.FileManager1.ShowItemProperties would open a separate
dialog for each file in the passed array.  However, selecting multiple
files in Thunar itself, and then selecting "Properties" from the context
menu would open a single properties dialog showing all of the files.

This makes the D-Bus method behave the same as the Thunar UI.

MR !309