Feature request: line mode

[cbeddow]

When line mode is on, then each double click on image/map adds a new node as part of a continuous line.

The tagging options will be disabled until the line drawing is complete. There should be a "Finish" button appearing.

When drawing is complete, then the tags will be filtered to only show line options.

Use cases:

1. Sidewalks
2. Bike paths
3. Power lines