silx.app.view.CustomPlotSelectionWindow : Add dataset directly from silx + drop overlay on plot

[RebeloAlexis]

Added functionality to enhance the user experience.

1. Added functionality for adding X or Y datasets directly from the main page

To do this, simply :

• Open silx view
• Select a file
• Right-click on the dataset and choose one of the 2 options (see image)
  • Set abscissa to plot selection
  • Add ordinate to plot selection
    • The "Plot Selection" page will open

2. Add user feedback when dropping on the plot

[t20100]

Great improvement! I'll merge once CI as run