Store user watermark text as a user preference

Reported by @okken [on Twitter](https://twitter.com/brianokken/status/1228107258556383238)

At present, the default slide deck includes a watermark that says "Made with Podium".

The text can be modified by editing the slide stylesheet; however, this is something that end users will almost always want to customize.

At the very least, editing this text should be documented; but ideally, it should be user configurable - either as a user preference, or as metadata associated with the slide deck.

This is effectively blocked on beeware/toga#90, as we need somewhere to store the user preference.

Start by reading the slide stylesheet where the default “Made with Podium” watermark is defined, then review the dependency on beeware/toga#90 for user-preference storage. Done means the watermark can be customized through a user preference or slide-deck metadata, with the editing method documented if configuration is not yet possible.

Written by the indexing model from the issue text.