GUI: replace .ui files

We still have these .ui files:

./GUI/View/MaterialEditor/MaterialEditorDialog.ui
./GUI/View/Toplevel/ProjectsView.ui
./GUI/View/Toplevel/SimulationView.ui
./GUI/View/Instrument/InstrumentLibraryEditor.ui

They define geometry, label, tooltip etc for GUI widgets. For many other widgets these properties are set through C++ statements.

We should unify our GUI programming style. The preferred style is direct C++ coding. We should get rid of all the .ui files. Advantages:

• Uniform style facilitates browsing of the code.
• Direct C++ coding allows us to use core functions like className().
• Direct C++ coding allows us to unify the graphical layout by using constants, inheritance, or other programming constructs.

added GUI cleanup labels

mentioned in merge request !1640 (merged)

changed the description

mentioned in issue #601 (closed)

changed the description

assigned to @a.nejati

mentioned in merge request !2148 (merged)

mentioned in commit 1cfa99b7

closed with merge request !2148 (merged)