Change the TracksViewer not to be singleton

[cmalinmayor]

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[cmalinmayor]

New idea: No more singleton tracks viewer. One napari viewer, one tracks viewer per tracks.

This creates a problem with the keybinds. Now we need to specify the keybinds only apply to the TracksViewer that is "active" (similar to how napari does it with layers).

We have one TracksList that is not within a tracks viewer, it is a core application data structure. This can be a singleton related to the napari viewer, since there should only be one per application. The TracksListWidget can live in the application_menus directory. The actual TracksList data structure can just be in main directory. Each element in the list is associated with one TracksViewer (can be created on click or just make them when you add to the list).