Create PersonStorage

• Save people in PersonStorage not in Tracker
• Move methods from Tracker to PersonStorage
• Add new methods needed

This PR touches a lot of functionality. So essentially, we should test all shortcuts. Also, merging of trajectories. Usual tracking and recognition should be covered by our regression tests.

If we check all shortcuts, we can probably also close (we cannot, since there is an Tracker-unrelated issue: Holding left mouse button does not enable to move image) #141 (closed).

Most methods of PersonStorage are old methods of Tracker, with the same name. So if the diff doesn't work nicely, just open the new PersonStorage and the old Tracker next to each other (the methods should mostly have the same order as well).

Closes #223 (closed)