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