timeline: Make the Shift and Control keyboard mask variables private
timeline: Reimplement the seeking and framestepping keyboard shortcuts

This allows using left/right and shift+left/right to seek backwards or forwards
for one project frame or for one second.

Ctrl+left/right (seek to the nearest clip on the left/right) was not
reimplemented as this is a buggy and rarely used feature that complexifies code.
utils/pipeline: Prevent tracebacks when holding left/right keys on the timeline