Add a layer-table-plugin with a list of segments

[cudmore]

Would be useful to add a layer-table-plugin with a list of segments along with some calculated columns for each segment.

If you grab the layer table from GitHub, the current working branch is cudmore-labels-oct-30. The workflow build might be failing but should work reasonably well if installed from source.

Columns could be:

Idx : int
  Segment ID
n : int
   Number of 3D points in the segment
umLength : float
   Sum of euclidean distances between points in the segment

Interface

• When the user selects a segment in the list, all points in the segment should be selected in the viewer.
• When a point in a segment is clicked in the viewer, the table should highlight and snap to that row. May be hard to do so low priority.

Example from PyMapManager desktop

Here is an example from the PyMapManager desktop app. The list on the lower left is a list of segments, one row per segment. When segment 0 is selected all points in the segment are highlighted in the interface (cyan).