fw/ui: Save custom colors of QColorDialog in settings

[juli27]

Resolves: #30273

Save & restore the custom colors of the QColorDialog. Qt versions before 6.9 saved these per user in a QSettings instance under the organization name "QtProject". This functionality must be replaced by client code (us). Legacy values are read and used as default values.

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[DmitryArefiev]

Tested #30273 on Win10, Mac13.7.8, LinuxUbuntu24.04.3 LTS - FIXED