Prevent DBus initialization in strict sandbox mode

Depends on girara!8 (closed) and !67 to be merged first

Fixes #276

In order to avoid a dbus connection when using zathura in strict sandbox mode, we need to check the sandbox configuration option before gtk_init and set the dbus session bus address to "disabled:" in order to prevent the initialization by gtk.

This solution is reliable and works across dbus implementations according to the dbus maintainer. (This solution is also currently used by chromium) https://gitlab.freedesktop.org/dbus/dbus/-/issues/429