mark TODOs in code to make that happen.
Actually, it is not hard to do so, it just requires to combine
all the existing building blocks. When this is done, we can define
the "Session" subsystem as prerequisite for "GUI" in main.cpp
Unless I've made some (copy-n-paste) mistake with defining the facades,
this should be sufficient to pull up "the Session" and automatically
let the Gui-Plugin connect against the SessionCommandService
lib::Depend<TY> works as drop-in replacement for lib::Singleton<TY>
This changeset removes the convoluted special cases like
SingletonSub and MockInjector.
