[Popover] Popovers should expose a maximumHeight property

[Alberto Mardegan]

Currently, the height of a popover seems to depend solely on the height of its contents. However, there is no height limit, so if the contents are taller than the application's window, they will be cut out of the screen.
The developer cannot know whether this will happen or not, because different devices have different screen heights (also, there's the portrait vs landscape scenario), or the popover's contents can be dynamic.

The Popover object should expose a read-only property, maximumHeight, which the developer can use to restrict the content's height (for example, using a Flickable). The maximumHeight property must be computed so that it's possible to dismiss the popover.

[Zsombor Egri (zsombi)]

The content width/height should ideally be clamped to the maximum screen size minus some margin value. Instead of adding more properties aliased from the content itself, it would be wiser to expose that component through a property so developers can configure the content item freely.