kcm: Couple of QML nitpicks
Remove redundant overlay binding
They literally don't bring anything. Even the existing bindings are passing down the TimeDurationComboBox's own attached QQC2.Overlay.overlay, which is exactly the same as querying it on root from inside the component.
Explicitly specify arguments in signal handlers
Injection of parameters into signal handlers is deprecated. Use JavaScript functions with formal parameters instead.
Fix TextMetrics width hack with a proper advanceWidth metrics
Port LayoutMirroring.enabled to Control::mirrored
The mirrored property is shorter to write and, most importantly, more granular and the most correct of all the ways to reverse a layout.