Tutorial: Mouse cursor, painting fixes; clean up widget objects

[ferdnyc]

This fixes the issue reported in #3212 where the mouse cursor displayed while the tutorial is open would either be the Forbidden symbol (inadvertently set for dockTutorial in the UI file) or some seemingly-random resizing cursor. We now explicitly setCursor(Qt.ArrowCursor) in the tutorial QWidget.

It also cleans up some other aspects of the tutorial code:

1. The QWidget objects for closed tutorial popups are now cleaned up, previously they were building up and hanging around until the application terminated
2. Painting of the dialog is streamlined by using a pen and brush, instead of stacking two filled rectangles
3. A bunch of single-use variables (data copied into them, then referenced exactly one time) are eliminated by referencing the original source of the data directly
4. The tutorial data structure is reformatted for readability

Fixes #3212

[ferdnyc]

One more: Turns out drawPath() and fillPath() are redundant — fillPath() draws the filled path (without its outline), whereas drawPath() will draw the outline of the path (using the current QPen), and also fill with the Painter's current brush.

[ferdnyc]

I'm going to merge this, because:

1. It fixes the reported bug
2. I'm a tiny bit worried that some of the drawing changes may not work correctly in really ancient Qt, so I want to test the resulting AppImage. If it breaks, I'll revert.

[ferdnyc]

2. I want to test the resulting AppImage.

I did, and all is well.