BasicAppletContainer: Don't instantiate blur background when disabled

BasicAppletContainer: Use applet property instead of contentItem

From static typing point of view neither contentItem nor applet props actually have effectiveBackgroundHints property which belongs to the AppletInterface class. But it's somehow closer than accessing it via QQuickItem *contentItem. Also, how did it even work without qualified access in children components?

BasicAppletContainer: Port away from plasmoid context property to attached Plasmoid

BasicAppletContainer: Use strict === JavaScript equality operator

BasicAppletContainer: Refactor blurEnabled code

Turning it into a binding allows receiving updates when GraphicsInfo API changes, and now it also includes a dependency on effectiveBackgroundHints.

Overall, this patch frees opacity mask (background blur) from a dependency on appletContainer.applet object, so it will be easier to factor the whole mask out as a component.

BasicAppletContainer: Don't instantiate blur background when disabled

Basically, move the code into a Component, and dynamically manage it.

Test plan: switch between Breeze and Arc Dark themes in Plasma Style KCM. Use GammaRay to observe that blur component is being dynamically created and destroyed.