Use repaint-call in updateImage to mitigate flickering

QGraphicsScene::update schedules a redraw. Since we continue processing, we want the redraw to happen immediately. If it does not, we might run into flickering.

Using QWidget->repaint() should solve this issue. But since it is not easily reproduced, we cannot be sure it is fixed. However, till it occurs again, we regard the issue as fixed.

Closes #247 (closed)

Reviewer Checklist

Formatting

• the pre-build checks succeed

General code quality

• naming conventions are met (see .clang-tidy for detailed information)
• no static analyzer warnings in new code parts (e.g., use clang-tidy for checking)

General usability

• old versions of pet-files are still loadable

Only if changes in UI

• new elements are also saved and loaded from pet-file
• check if tab order is still correct
• all new SpinBoxes are promoted
• new keybindings added to Petrack::keyBindings()