panel: Fix xfce4-panel-CRITICAL when already running

`panel_dbus_service_is_exported()` is not the right test to know if an
instance is already running. Instead, let's follow the documentation of
`g_bus_own_name()`:
* if `panel_dbus_name_lost()` is called and `connection == NULL`, there
  is a dbus issue;
* if `panel_dbus_name_lost()` is called and `connection != NULL`, the
  name can't be acquired, which means that there is an already running
  instance if `G_BUS_NAME_OWNER_FLAGS_ALLOW_REPLACEMENT` is not set
  (which is currently the case for xfce4-panel);
* `application != NULL` if and only if `panel_dbus_name_acquired()` is
  called, if and only if `panel_dbus_name_lost()` is not called.
2 jobs for !43 with fix-critical-warnings in 4 minutes and 45 seconds (queued for 2 seconds)
merge request