Use the new completion features

[FichteFoll]

In order to support the new completion features, I'll likely make the next or the following build ST4-only. Very useful new features like #289 might get backported to an st3 branch, but I don't think there would need to be much maintenance for that branch otherwise. PD overall seems very stable with few bugs and most of the issues or PRs I get in are feature requests. I would then decide on a case-by-case basis if it's worth backporting.

For clarification, this includes sublime-completions files and plugin-provided. Especially because of the latter, I should probably do #289 first to have it backwards compatible.

[FichteFoll]

As an update, this is almost done. What's missing are theme and command completions, which I hope to finish tomorrow and then roll out the new release.

[FichteFoll]

Settings & command completions are done as well. Just theme (and scheme) left.

And a command to prevent buffer completions for keys that we provide completions for through .sublime-completions files.

[FichteFoll]

All completions should use this the new format now, both plugins and completions files.

Unfortunatly, I wasn't able to inhibit buffer completions. You can see my attempt at 83de08a, but it doesn't work as expected. Moved that to sublimehq/sublime_text#3643.