Make all subwindows dockable, and initial state is docked (consider tabs?)

[lars-petter-hauge]

It is not uncommon that a user will open multiple ERT applications with different config files. It is very easy to loose track of which subwindow belongs to which ert application. Making the subwindows dockable (and have them docked by default) should help on this.

Would it make sense to also make it possible from the main window to dock subwindows? Instead of looking for a "Plot" window and search for which ert application that will allow you to dock it.

[sondreso]

Not relevant after #8703