should respect XDG_DATA_DIRS to look for .desktop file

Binary package hint: indicator-messages

So, here is a testcase that unity respects and indicator-message doesn't:
 we have an evolution.desktop file in both /usr/share/applications (launching evolution) and /usr/share/une/applications (launching evolution express)

XDG_DATA_DIRS is set to:
XDG_DATA_DIRS=/usr/share/gnome:/usr/local/share/:/usr/share/
on gnome session
and
XDG_DATA_DIRS=/usr/share/une:/usr/local/share/:/usr/share/
on the une session.

Both have the messaging menus bits to show in the indicator, but only /usr/share/applications/evolution.desktop is taken. From the XDG spec, we should cycle on XDG_DATA_DIRS and stop on first of each .desktop file we find. (so /usr/share/une/applications/evolution.desktop should override /usr/share/applications/evolution.desktop).
Consequently, we will launch from the messaging menu evolution express on UNE and evolution on the messaging menu.

The current infrastructure with /usr/share/indicators/messages/applications/<filename> should be so deleted to rely on scanning the .desktop file.

The current consequence (for now) is that in une, you have evolution express (three desktop file), that you can run from unity, taking evolution in the express mode. But if you launch it from the messaging menu, you will only have the full evolution environment which isn't suited for netbook.