Replace QMap with pointer-type as key with QHash

Not using a node-based associative container is more memory efficient,
and comparing pointer values (rather than the pointed-to-objects) using
operatore<() is not producing a meaningful ordering anyway.

Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV164
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV165
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV344
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV345
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV542
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV547
Task-number: QTBUG-146050
Pick-to: 6.12
Change-Id: Idd302c3a6243b824b5e8e390d3dabf3085317ae7
Reviewed-by: Kwanghyo Park <kwanghyo.park@qt.io>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>