Delay maximize button click to next event cycle

Summary:
The delay to next cycle dance is needed for Aurorae. Maximizing a
window can result in the decoration being destroyed, in which case
QtQuick can trigger a crash.

A test case is added to simulate the situation and ensure that maximize
still works also after the change.

BUG: 362772
FIXED-IN: 5.6.5

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1586