Add Play button?

[peteruithoven]

A lot of video torrents are actually folders with one or more videos. While I assume you wouldn't want to change the default behaviour of double clicking it might be interesting to add a play button. This could then open the file in the user's default video player.

Webtorrent desktop for example seems to open the first "Playable" file:
https://github.com/webtorrent/webtorrent-desktop/blob/master/src/renderer/controllers/playback-controller.js#L46
To determine whether something is playable it compares the extensions with this list:
https://github.com/webtorrent/webtorrent-desktop/blob/109accdb541c5220b1e3cd856b161d4ba59def1b/src/renderer/lib/media-extensions.js

[peteruithoven]

In the PR I've only added a "Play video" to the context menu, but I'd like to include it in the main list somehow. Some options:

1. Adding a play button on the right. Downside is that it's probably confusing with the current play pause button.
2. Adding a play button on the left, next to the main icon. Downside is that it would break the layout, making it harder to quickly scan all the titles.
3. Adding a play button on top of the main icon. This might get a bit busy though. Some options: