appDisplay: Return an invalid page for coordinates outside the scroll area

While this changes are triggered by an extension need, I feel that it can be just a generic fix, avoiding further monkey patching from the extensions side.

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When using extensions such as DashToDock and using them in the left side of the display (in an rtl world, or in the right side otherwise) drag events of AppIcons on the dock itself may be stopped because the shell considers such event as a drop to a "previous" AppView grid page, and when we're already on the first page, there's no other previous page to drop on.

However, in presence of another widget before the appview, this assumption isn't valid anymore.

As per this, when getting the page per coordinates, only return a valid page if we're hovering the scroll area, as any other event outside this doesn't make sense to be tracked at all.