Support multiple entries in XDG_DATA_DIRS

Summary:
Multiple themes named "Default" were inserted into the QMap,
one for my prefix and then one for /usr/share, which replaced it.
What we want is this to be a single theme with multiple search paths.

Test Plan:
This fixes the "Kontact introduction" in my developer
setup where everything is installed into a custom prefix.
Before this fix, only /usr/share/messageviewer/about was looked at,
and I don't have kontact installed there.

Reviewers: dvratil, mlaurent, vkrause, skelly

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D29613