fix virtual keyboard on Plasma Mobile

Summary:
the newly introduced resize button shouldn't be visible on the phone and
for some reason using a ToolButton prevents the qml to load at all on
the phone (the problem seems right into the C++ part of ToolButton as
is broken on every QQC2 style)

Test Plan: work again on phone, still work on laptop

Reviewers: Plasma, bshah

Reviewed By: Plasma, bshah

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17483