LibreOffice wants to open URLs by calling /usr/bin/xdg-open, which the flatpak
org.gnome.Platform runtime does not provide.  However,
<https://github.com/flatpak/xdg-desktop-portal> will provide a Desktop portal
through which an application can make an OpenURI request.  So the idea is to
include a fake xdg-open application in the LO flatpak, that will use gdbus to
call that portal.  However, I see no way to make the LO flatpak provide that as
/usr/bin/xdg-open, so instead provide it as /app/bin/xdg-open and hack LO to
use that path when run under flatpak (where the LIBO_FLATPAK environment
variable is already used for the OfficeIPCThread).

Change-Id: I63bc0bfb937ef767958ba4d34b0c3653e38ec868