Coding style and naming in AkRanges library

Summary:
Fix coding style to follow what we use everywhere else and make the code
more legible. Also rename InContainer to RangeLike to better distinguish
between templates that only accept containers and templates that accept
Range or a container.

No change in functionality.

Reviewers: #kde_pim, vkrause

Reviewed By: #kde_pim, vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21453