desktop: open a file via xdg-open without additional output window

desktop: open a file via xdg-open without additional output window

Currently there is a difference between opening an additional data
file via the Edit->Open menu or by opening a file via xdg-open (same
for doubleclick on Filebrowser). In the latter case the on_open
callback is called which runs an additional post_initialize. That
results in an additional output window.

I changed the behavior such that post_initialize is only called
when there is no psppire window yet. That happens when psppire
is started from a terminal with the file as argument on linux.
Further I check if there is an empty datawindow and replace that
in the same way as it is done via the Edit->Open menu.