So it can be implemented by SliceWindow

and using a static loading mechanism. This is so that we can revert to
using default constructors. Moved away from using the dynamic factory
class and instantiators as they cannot presently support this type of
instantiation mechanism.

Folders are a little special, so I've decied the code would be better
off in the ProjectSerialiser class. This is more consistent with how
loading works. ScriptingWindow now properly conforms to
IProjectSerialisable, with a small workaroudn for a MantidUI use case.

The list of open files in the window is a list of absolute paths so
the new path needs to be made absolute before checking if it is open.
Refs #15682

The behaviour is then consistent whether the file was already open
or not as the currently active tab is the requested file.
Refs #15682

Refs #13417

Refs #13265

Refs #13265

It is only implemented for the Python environment.
Refs #13265

Refs #13082

Refs #6453

Refs #9270

Refs #9059

Refs #2522

This allows it to be saved more easily and is probably what would
be expected.
Refs #2522

The menu was disabled but the actions were left enabled so the
shortcuts were still alive.

Also tidied up the execution code so it's not so cluttered.

The API bits need a some tidy up but they are at least there and
can be searched.