Add filter functionality and UI

[eppfel]

As discussed in the sidebar menu redesign: #556 and the search #558 , we need to implement a powerful filter into the Android App.
The following filters came up, so far:

• "on your device" (to get rid of the menu item/view and integrate as a filter)
• By Media / file format
  • photos for a starter

We have to come up with a plan how to integrate filters in the UI.

[eppfel]

The straightforward solution would be the 3-dot menu, but that is too hidden.

@mario proposed a footer #556 :

I think the menu is becoming overwhelming as-is anyway, and you need somewhere to show how is the list being filtered (and footer is a good place to put it).

I would rather introduce a second bar on the top, than a footer for this. And we have to think about the amount of filters, because I fear we won't fit them horizontally anyway.

[eppfel]

And maybe this needs a stronger integration into the search functionality.

[eppfel]

Discussion on sort is in #340

[eppfel]

UI consideration have to include landscape mode (#38)

[mario]

@eppfel search api on the server should definitely include all the filtering options we want in the app.

[eppfel]

Yes, sry. I meant "integration" from UI point of view....