FunnelModel: fix invalid model API usage.

Summary:
The new asserts added in qtbase in commit 00c09e752f (Qt 5.11) are triggered
by this invalid code, which sets up the model before the begin* methods.
So Qt already sees the new rowCount in that method, which is wrong.

Test Plan: Opening the K menu

Reviewers: hein, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12275