introduce layer oriented sample editor

See merge request !413

introduce WidgetMoverButton (to realize the "move layer" feature in the upcoming layer oriented sample editor)

More preparations for layer oriented sample editor

See merge request !412

Overhaul UI: simplify displaying the results of jobs

See merge request !410

Add the possibility to show the empty widget in ItemComboWidget also for other items (e.g. running jobs)

Add a blank widget to ItemComboWidget for the case that a nullptr SessionItem is set as the current item (instead of the show/hide logic)

Merge remaining funcionality of JobOutputDataWidget into JobResultsPresenter and delete obsolete JobOutputDataWidget

Remove ItemStackPresenter (which was only used in "single widget mode" anyway) from JobOutputDataWidget

Descriptors3

See merge request !409

CMake: Remove unnecessary dependencies on C++ Boost libraries

See merge request !408

`Base`, `Fit` and `RootMinimizers` have only a dependence on the Boost header
files. No Boost libraries are needed.

Overhaul ui: small improvements

See merge request !407

Fix warnings

See merge request !406

Introduce sample selection list

See merge request !405