session properties: show running apps that can/will be saved on exit

[monsta]

Since not all of the running apps can be remembered for starting in the next session (see also https://github.com/mate-desktop/mate-session-manager/issues/42), it might be handy to show the list of apps that can/will be saved on exit. Xfce does that in their xfce4-session-settings app:

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[flexiondotorg]

Good idea.