Better use of app pinning, remember prior downloads dirs

Keep a history of (unique) directories used as downloads dirs. Access that thru a popup triggered by clicking the DownLoad dir item in the Add Books page. Clicking on a history entry changes directly to that entry. New entries are added from the popup, and entries can be deleted from there by clicking on a delete item entry.

This goes with a web page change on how to install downloadable books from OverDrive and rbDigital and the long path to the directory OverDrive uses for that.

The emulator doesn't behave exactly the same as real hardware when doing Application Pinning for API 21-27. Get the change in 1.1.3 working for working for those APIs.

Fix low-contrast text when Title Edit cannot proceed due to an illegal name.

Minor bugfixes.