Adding plasma5 support for QKdeTheme

Plasma 5 uses the same config files structure, but follows XDG spec
instead of storing them in custom locations.
Added '5' to supported KDE session version, and passing it to other
methods because 'kdeglobals' is located directly under XDG config
dirs and not deeper inside.

[ChangeLog][QtPlatformSupport][QKdeTheme] adding plasma5 support

Change-Id: I5f1d8d8f8d02449328db3ffc391ad23506cd67c7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>