Allow to view overlap regions in region list dialog

[iromeo]

Context Menu | Overlap table feature makes a table with paired overlap of tracks (see screenshots below). The dialog calculates overlap regions and reports their number, but we cannot easily view the regions in overlap. So it will be useful to implement a feature that:

Show a context menu for cell, that allows to:

1. Load overlapped regions into regions list dialogue (Add region selection tool #250)
2. Add overlapped regions as a new track into the browser. I.e. filter regions from one track that overlaps other regions & export to the file.

P.S.: Not sure if we should implement only 1 or 2 or both. When 2 is implemented, the user will be able to show new track regions in the region list dialogue using the existing feature. The drawback - too many mouse clicks in order to only view a couple of examples from the overlap if a separate resulting track is not needed.