Tidier playhead controls

The time panel has a dedicated "end of timeline" button next to play/pause, so jumping to the latest data and following it as it streams in no longer requires the playhead navigation menu.

The playhead commands also read more clearly: stepping between logged data is now labelled "Previous event" and "Next event", and the fixed-size jumps say how far they move ("Backward 0.1s", "Forward 1s", …), including what they do on a sequence timeline.

Going to the beginning/end of the timeline moved from Home/End to Cmd+Shift+left/right arrow (Ctrl on Windows and Linux).