Always use whole image as boundingRect fo Coord

The problem seems to be the boundingRect of the CoordinateItem, which tries to be as small as possible and results in this error. We might want to just set the boundingRect to the image size, as we do for most other widgets, since this should not be very performance intensive (i.e. we do not have a lot of small individual objects, where culling is important)

Closes #313 (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()