WIP: add Smartspacer support

[pacjo]

when ready, will close #771 .

So, a few things. While there is basic support here, it's not ready for merging, hence the draft. Also there won't be any new commits from me for probably 2 or 3 weeks, since it's summer and I'm not taking my pc with me. If someone wants to pick this up during that time, it'd awesome.

...and there are still some issues to iron out, so a TODO:

• [ ] fix touch input, since currently dragging pages requires a two-finger swipe
• [ ] add proper icon in widget picker
• [ ] make background toggle-able
• [ ] resuse smartspacer sessions / view (just don't get a new one each time)
• [ ] don't hardcode widget size
• [ ] (probably) correct padding all around
• [ ] (probably) fix strings.xml, since it's the first time I'm working with translations on android
• [ ] also add an option to disable page limit, as support was added just the other day