This is a highlevel library for working with xdg-app without using
the commandline interface. The primary usecase for this is for
creating a graphical frontend for app installation/update.
Some xservers out there (like xorg 1.17.1) have a broken server interpreted
local xauth, which causes apps to fail to connect to the xserver.
This fixes that by propagating Xauthority data such as the MIT-MAGIC-COOKIE-1.
This moves a all source code into separate subdirs per binary. The
helper and the generic stuff goes into lib/ which is then used by all
the others. For now this is a completely internal library, but at
some point we will probably clean it up and expose some subset.
Also, we move the dbus proxy to libexecdir.