Reorg dirs: disentangle widgets in `GUI/Views`

To disentangle some dependencies in the GUI/Views directories, the following has been made:

• GUI/Views/FitWidgets/plot_constants.h has been moved to GUI/Views/IntensityDataWidgets/. This way FitWidgets is dependent on IntensityDataWidgets but not vice versa anymore.
• GUI/Views/SampleDesigner/DesignerHelper.* has been moved to GUI/Views/CommonWidgets. This way GUI/Views/InfoWidgets is not dependent anymore on GUI/Views/SampleDesigner.

Furthermore GUI/Views/StyleUtils.* has been moved to GUI/Views/CommonWidgets/ because general GUI-related code is located there.