Make it possible to insert points in add_line

[raacampbell]

In "regular" add mode, new points are appended to the end of the list. This is is what currently happens. This issue is to make a new option to add mode that should be have as follows.

• User right-clicks on a point and it becomes highlighted 6e69a0e
• The next point to be added is inserted into the points list before this point 6e69a0e
• To make this clearer, we can at this stage add a new line in a different colour that updates with mouse move. So is flexible and awesome. This line will link the highlighted point to the existing point before it in the line via the mouse cursor.

[raacampbell]

Would be nice to do the third point, but currently the feature works.