Allow Creation of New Sinkhole Points

[mradamcox]

Drawing should be implemented to facilitate the creation of new points. Initially, these can be new Sink objects, though in the future the same utility may be used to create other objects.

[mradamcox]

New thinking is that adding a "create feature here" button to the right-click context menu that shows lat/long etc. would be a good way to implement this feature. Clicking the button would open the existing "sink update form", one key difference being that the Sink ID, elevation, and depth fields would be "N/A" or "--" because the feature hasn't actually been added to the database yet.

This strategy would work best for point features, but lines and polygons could also be created if a separate button created for each one. In the case of lines/polygons, subsequent clicks would add vertices until a double-click finished the feature.

Multi-part polygons or lines need not be supported at this time.