Offline usage support

[joakim-olesen]

@R0tenur, please add some explanation here

[R0tenur]

Longer sessions.
In everyday use apps you don't get logged out every hour.
Also, when using on phone your internet connection may not be that stable all the time. WOuld be great to make it possible to use photo garden offline.

• Cache current state, what filters, what sort and so on. Recreate when changeing state and you have avalible connection, else use some kind off offline fallback functionality. Sorting of already cached photos etc.
• Make PG installable, to make it possible to use from your phones home screen.
• Changes, for example editing of photos and in the future manual tagging should store the changes in cache and send it to the server once you got connection again.