Use styleHint to enable opening an item on single click

Connecting both clicked and activated signals causes unnecessary reloading
of KCMs when clicking files or folders is set to open them, and will
crash systemsettings when opening Powerdevil settings.

This commit uses QProxyStyle to set style hint in CategorizedView.
`QStyle::SH_ItemView_ActivateItemOnSingleClick` now always returns 1
regardless of the behavior settings, so an item will always be opened on
single click.

CCBUG: 443858