QKdeTheme: use system-wide kdeglobals as a fallback

Determine KDE prefixes in the following priority order:
- KDEHOME and KDEDIRS environment variables
- ~/.kde(<version>)
- read prefixes from /etc/kde<version>rc
- fallback to /etc/kde<version>

Task-number: QTBUG-36184
Change-Id: I9010ea485f1954b21bda73b02993dbddef67eb1d
Reviewed-by: Dominik Holland <dominik.holland@pelagicore.com>
Reviewed-by: David Faure <david.faure@kdab.com>