Setting playback speed influences exported framerate in trajectory files

This MR separates the playback fps from the original sequence fps.
User can set the sequence fps via the file menu and it is shown in the window title.
Sequence fps and playback fps are exported now.

Closes #332 (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()