Allow drag n drop & fix a few issues with file sharing

[oakkitten]

See individual commits.

For drag n drop, the entire buffer fragment acts like a drop target.

I'm not sure whether or not we need drag-n-drop indicators, like the ones you see in Gmail. One one hand, we normally only show a single buffer. On the other, it might be somewhat useful to indicate that the buffer list or the title bar are not targets for dropping files. I decided to not do any indicators because I am lazy and who even drags and drops files on Android anyway.

[oakkitten]

Please review, paying attention to the last commit. Coroutines can be deceiving