Visualization buttons
3 changes (each in its own commit), all related to the visualize tracked trajectory part of the GUI:
- added buttons to go to the previous and next frames, right next to the 'jump to frame' button.
- added a button to manually remove data points and set them to nan. This goes right under the manual head/tail swap correction button.
- I've tried to make the frame slider as long as possible (while leaving everything else the way it is). To do that I've added a line-break to the 'start/stop playing' button so it takes up less horizontal space, and extended the length of the slider. The button does look uglier (on mac) because of this change, but having a longer slider is worth it I think.