Allow syncing with an external XML file

[Victor239]

Currently we can export a XML file or import one, but we can't opt to continuously sync with one. Having this feature would enable utilising the same XML file on both mobile and desktop, using Syncthing or another app to sync between devices.

Workflow:

• An option in Settings to select a XML file to stay in sync with. At this stage the user is asked whether they want to for this first sync either (1) overwrite the external file using the app's database or (2) overwrite the app's database using the external file.
• After this first write the app then checks the timestamp of when the external file was last modified and stores it in the database.
• Whenever GnuCash Pocket goes into the foreground it should check whether the file has a different modification timestamp than what was last recorded in the db. If so, then refresh GnuCash Pocket database with data from the XML file.
• Whenever making edits inside GnuCash Pocket aim to update the XML after 1 minute of inactivity within the app. After updating the XML then record the last modification timestamp of the file to the database.

This process will then allow the user to seamlessly swap between mobile and desktop whenever they wish.