Better handling of multiple open data files in the GUI

[tohojo]

Presently, the GUI tabs represent open data files, but the feature to
combine plots can also load multiple data files in one tab. This is not
really visible, and there's only an "add more / remove all" granularity
to the files open in each tab.

Having a better way to manage which data files are open and which are
used in each plot (tab) would make flipping through many data files
easier.

[dtaht]

Being able to "Tear off" a tab into it's own window would be sometimes handy.

[tohojo]

Started on this by adding an 'open files' widget.

Could still be improved somewhat, but at least the open files are now visible...

[tohojo]

This has now been added :)