Add built-in documentation for SWORD module offline installation

[tobias-klein]

Ezra Bible App can use modules that are installed separately (also from offline sources). The app reads these modules from existing folders based on standard directories on the different platforms.

The locations/paths may not be entirely clear to the user and the approach for module offline installation should be somehow documented inside the app.

We could add a new button Offline installation in the settings menu underneith these existing buttons:

When that button is clicked we could simply show some instruction in a dialog that describes how the offline installation works and what's the applicable path on the currently used app platform.

This is based on discussion #971.