Autoimport completed class names

[nya3jp]

This patch introduces a new interface to represent the import order. We also provide its two implementations: the simple one (all imports in a single section with lexicographical order), and the one conforming to the Chromium Java style guide (which I use for my project).

The interface is used in two places: the existing logic to add an import as a quick fix, and the new logic to add an import on completing class names.

The style can be configured by user settings.

Fixes #96