Add support for loading and using an icontheme in a RCC file automatically.

Authored by dfaure on Jun 14 2016, 8:31 PM.

Description

Add support for loading and using an icontheme in a RCC file automatically.

Summary:
This generalizes the solution currently duplicated in kate and kdevelop,
to all apps.
The intended use case is e.g. BINDIR/data/icontheme.rcc on Windows.

Test Plan: Concept patch tested on Windows by Kåre Särs, this implementation tested by the unittest

Reviewers: sars, cullmann

Reviewed By: sars, cullmann

Differential Revision: https://phabricator.kde.org/D1878