TrackScheme advanced filtering of visible lineages

[xulman]

When the tracking data is large, technically speaking when there are many roots/trees, the content displayed in TrackScheme is "very wide". Assume one adds a solo spot, which appears on the very right of the view, and wants to link it to a tree on the very left... the linking (key 'L') must happen fully inside the window view, but to display the far-left and the far-right at the same time requires zoom-out view... when spots are potentially no longer identifiable/visible/distinguishable.

A particular "workaround" solution could be to reduce the amount of displayed roots/trees. But how?

An idea is to use the "context:" feature of the TrackScheme:

which could allow a user to specify which lineages to have displayed...
(note that BDV context cannot be used for this as the link requires spots at different time points and thus impossible to display in BDV at the same time).

Not sure how GUI-way to achieve that, thinking of "list of currently visible trees" which checked checkboxes, unchecking is to remove the item, adding a new item could be a separate list item "new..." which would trigger a dialog like, e.g., this one:

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(this is potentially related to #162)