Use non-deprecated <gui> root element

Summary:
The DTD talks about <kpartgui> being deprecated as toplevel element,
so the data and tests should follow that.

Also fixes wrong DOCTYPE name usage, "gui" instead of "kpartgui"
makes no sense given there is only the one DTD
(will be separate commit).
To reduce confusion there either should be a new DTD
or the already existing kxmlgui.xsd schema and the respective
header should be promoted.

Test Plan: Tests still work, tested apps also.

Reviewers: Frameworks, aacid

Reviewed By: aacid

Subscribers: aacid

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5296