Make SessionSettings derive from UiSettings

Make SessionSettings derive from UiSettings

In order to avoid code duplication, we'll want to be able to put mainwin state
saving/restoring into methods taking an UiSettings object. Hence, we'll let SessionSettings
inherit from UiSettings rather than ClientSettings, and virtualize the accessors.

Also kill the separate sessionsettings.* files, as we want to centralize the various settings
objects into common files.