xfce4-session: New Xfconf property "ShowSave"

[Akbarkhon Variskhanov]

Hi.

Some weeks ago, I created an MR for a minor cosmetic change to the
logout dialog[1]. There was no response so I thought maybe no one
noticed.

I added the following line to check for the new property:
> && xfconf_channel_get_bool (channel, "/general/ShowSave", TRUE))

I locally packaged `xfce4-session` with this change and have been
using it ever since. The logout dialog is just perfect to my taste
now. Maybe I'm not the only one who finds the save session checkbox
annoying, so I thought I'd share the patch.

What do you think?

[1] https://gitlab.xfce.org/xfce/xfce4-session/-/merge_requests/29