Add webdav support or other cloud to synchronize the beancount file

[cc9014]

[johannesjh]

I think that using the new android APIs, as suggested in #6, could also help to solve/improve file syncing.

taking nextcloud as an example:

• nextcloud's android client provides intents for choosing files, see e.g., https://github.com/nextcloud/android/issues/3441
• if beancount-mobile and/or upstream @nativescript/imagepicker uses a more modern file picker APIs as suggested in #6, this could allow to choose a file that nextcloud will keep in sync (as opposed to the behavior that I am currently experiencing, i.e., when using beancount mobile v1.5.0, the file picker does not let me choose files from the nextcloud client app)