Polish plugin print workflow demo

[caseycesari]

Overview

Makes a few changes to ensure that the plugin print demo developed in the identify point plugin is more useful to plugin developers. These changes came out of discussions with @cschneebeck.

See commit messages for details.

Connects #1113

Demo

Testing Instructions

• Open the identify point plugin and click the print button.
• Move the print preview map to somewhere notable that you can remember.
• Add the sample layer, and ensure it shows up on the map (it's mainly along the eastern and southern coast).
• Click the print button.
• Ensure the printed map center and extent roughly matches the extent of the print preview map.

[kellyi]

Should this PR be targeted at master and not develop?

[kellyi]

->

[caseycesari]

Thanks for the review!