Store preferred screen orientation as a user setting

See #48 for the original report.

If you have your laptop configured so that the external screen is the primary, Podium will display the slides on the screen you expect. However, if you have your laptop set as the primary, it won't, and you'll need to switch the slides and notes every time you start Podium.

This should be user configurable. Once you've switched orientation, that orientation preference should be saved as a user preference, and when you start Podium in the future, that preference should be read and preserved.

This is effectively blocked on beeware/toga#90.

Read the original report in #48 and investigate how Podium currently switches slides and notes when it starts. Check the available user-preference path and the status of beeware/toga#90 before proceeding; done means a switched orientation is saved and restored on future launches.

Written by the indexing model from the issue text.