Add a swipe-to-start feature

[shocklateboy92]

I originally wanted to add a "Continue" button on the session summary page (kind of like what Wanikani web has), but it turned out to be far too difficult to handle all the race conditions caused when starting a new session immediately after finishing one.

So this does the next best thing. It adds a SwipingScrollView to the MainActivity (the diff is giant because of indentation changes), and triggers the first available action (in the following order) when swiping right:

• Resume session
• Start reviews
• Start lessions

I also added a → symbol to all the buttons that can be triggered by swiping right. If you can think of a sensible place to write that down so users will be aware of the functionality, let me know. Otherwise, it'll just have to be a hidden gem :smile:

[shocklateboy92]

Okay, I've been using this for a few days and am quite happy with it now.

I ended up replacing the NestedScrollView in MainActivity with a SwipingScrollView. Having both seemed to cause no end of issues. It's now working much better with just the swiping one.

I'm not sure why that NestedScrollView was needed in the first place. Everything seems to be working fine without it. Everything that I can see at least - maybe when I get to level 60 there'll be some special feature that breaks without it :stuck_out_tongue: .

Anyway, I think it's ready for review now @jerryhcooke