a Custom Session Management plugin

Adds a text entry for each command in the plugin preferences (not very aligned, though - is there a way to fix that?).

The Gnome/Xfce plugins launch a dialogue for log out/shut down with with further options, so this commit renames the classes in session_support for these and creates extra classes for each individual command they each cover. (That is, log out covers log out and switch user, and shut down covers shut down, reboot and suspend.)

The system-switch-user and system-suspend icons don't seem to be in the freedesktop icon naming spec, but I have them on my machine - should they be changed?

The obvious thing to do to disable any of these is to empty the text box, but for some reason Kupfer doesn't save blank text entries (see ``kupfer.ui.preferences.PreferencesWindowController._make_plugin_settings_widget``).