[QgsQuick] Qt6 updates

[tomasMizera]

There were quite a few issues with qgsquick map canvas when building with Qt6:

• forward declaration no longer works when the type is used in Q_PROPERTY (see https://www.qt.io/blog/whats-new-in-qmetatype-qvariant#:~:text=QMetaObject%3A%3AinvokeMethod.-,QMetaType,-knows%20your%20properties)
• f55b838 failed to remove the QQuickItem::geometryChanged call that was outside of the ifdef - could not build the code with Qt 6.3.2

PR also removes unused imports

[nirvn]

Looks good

[nirvn]

@tomasMizera , let's backport to 3.28

[tomasMizera]

Sounds good!