Make the download button actually work

[KentoNishi]

Maybe provide functions for downloading text files and going fullscreen as native js interface methods?

[arkon]

For fullscreen, I can implement a JS interface method to toggle that.

For downloading stuff, would it pass strings of text to a JS interface method?

[KentoNishi]

For downloading text, I can just pass the string value (with newlines already included). For screenshots I can send a base64 encoded image url

[arkon]

Stubs for toggling fullscreen and downloading text added here: https://github.com/LiveTL/android/commit/05c8dc4c6211caf4524c650cab0face7511978e0

Let me know if the arguments aren't what you expected.

I'll follow up later on sharing images in #23.

[KentoNishi]

Perfect, I'll add it later

[KentoNishi]

@arkon small request: make toggleFullscreen accept no arguments. passing the boolean means tracking the FS state in the extension code rather than just triggering a toggle and that requires communicating FS exit events and such, so it would be much easier if it were just the toggle func. i'll implement it with that assumption in develop

[KentoNishi]

forgot another request: make the download text function accept the filename as the second parameter

[arkon]

Interface changes are done, and fullscreen toggling is hooked up (although not tested).

[arkon]

No longer relevant since not trying to use the extension anymore.