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