GUI improvements

Resolves #189 (closed)

This MR addresses:

• UI problems (see pictures below)
• boiler plate code
• minor issues and corrections

Fixing the UI problems is mainly achieved by correcting layout definitions (size policies & size constraints). Also the resizing of the settings pane was rectified: the horizontal scrollbar is now always hidden, the vertical one does enlarge the pane instead of hiding the contents of the pane.

Reducing the boiler plate code is mainly achieved by the introduction of GridFiller and PropertyScrollArea. At a few spots it has been done with lambdas or method extraction.

The class Spoiler had to be reworked because of various drawing/sizing issues.

The addressed UI issues are: