Sort documentation providers in documentation view

Summary: This patch performs sorting of documentation providers list in documentation view.
Unsorted list is ugly and blocks comfortable finding of documentation sections.

Reviewers: mwolff, KDevelop, kfunk

Reviewed By: mwolff, KDevelop, kfunk

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2461