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.
Closes #920 (closed)