Store Data offline and allow to create transactions without internet connection

[kayserlein]

Sometimes, you are somewhere with bad to no reception and still pay for things. It would be great to be able to check the most recently downloaded data instead of the app purging it on closing and then reloading it on next launch. This would help greatly to not lose the habit to immediately enter every transaction immediately after having executed it.

An optional background sync with Firefly that allows the app to keep the data up-to-date at a set interval chosen by the user would also be a great supplement for that

[dreautall]

Duplicate of #27. To give a short update: this is one for the "in the long run" list, as it would require massive changes to a lot of code.