Adapt configuration dialog to Qt6

Reason for the change

Adapt to some changes in Qt6 and KF6

Test plan

• Open config dialog and mobile sidebar
• Change color scheme in mobile mode
• Change a config item, then close the config window without applying the changes to see the config warning dialog
• Add a new music location to see the Folder Dialog

Screenshots or screen recordings

• Only visual change is the size of the mobile color scheme menu

Issues

• I am unable to see the mobile sidebar when built with Qt6 (see https://bugs.kde.org/show_bug.cgi?id=473214 ) (current workaround is to open the config dialog with ctrl+shift+,, but this means MobileSettingsPage.qml is untested)
• The mobile color scheme menu became chonkier (maybe not a big issue)
• The config warning dialog (which appears when trying to exit the Configuration Window with unsaved changes) is no longer a native dialog, and has lost support for setting a custom icon.