make Beatjump use seconds if no beatgrid is detected

[ronso0]

Feature Description

Make Beatjump controls switch from beats to seconds if no beatgrid is detected so these controls are of some use at least.
This is useful for beat-less tracks as well as jumping in long tracks while they're being analyzed.

• the beatjump size spinbox could show the s suffix
• I guess making the loop engine deal with seconds is too much hazzle, so the beatloop size spinbox and the loop activate button (with no saved loop) should probably be greyed out

[ronso0]

Alternative:
if track has no beats, use fake beats (100 BPM?)

[ronso0]

Fixed in #12961