Redline widget usability improvements

[jng]

The user workflow for the new redline widget takes significantly more steps than the previous redline widget. The usability can be improved somewhat as follows:

• When clicking "New" button on the initial page, prompt() for a layer name (must check name to guard against duplicate layer names). Once added, create a default redline layer definition, and add it directly to the map and redirect to the edit redline page. Do not direct the user to the page for editing the layer's style. We'll add a new button for that purpose (see below)
• Add a new "Edit Style" button to the initial page, allowing for the style of a redline layer to be modified.
• Repurpose the "New Redline Layer" page to be used for editing the style of an existing redline layer. Make the layer name field readonly to prevent saving to a duplicate layer name.
• In the edit redline page, add a checkbox indicating if the user should be prompted to enter a label for any recorded redlines. This checkbox should be unchecked by default.
• Add the ability to upload previously created redline SDF files. Just like the creating a new layer, uploading a redline SDF file will automatically add this layer to the map and redirects to the edit redline page for recording of redlines. We assume the SDF being uploaded has a compatible feature schema. If it doesn't we redirect to an error page indicating that the uploaded SDF is incompatible.

This set of changes reduces the redline management burden on the user, and makes all management functions optional instead of required.

[jng]

Also the Download button should be active for selected feature source *or* selected layer

[jng]

Fixed r2502, r2503, r2504 and r2505