include cleanup

This MR just deletes some includes or moved them into cpp files. The hope is that this leads to less files being included in each file (less includes -> faster compile) and fewer re-compilations on a single file change.

Some methods were put into the cpp files to allow for forward declaration in the header. Some includes needed to be added, because the files depended on transitive includes.

Reviewer Checklist

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
• documentation updated (if applicable)

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()