[hover] consider parents when detecting hoverable elements

[JanKoehnlein]

In the multicore example, the hover shows when you enter a core but disappears again when you enter one of its labels. This is kind of distracting.

It is because we only look at the element directly under the mouse cursor to trigger the hover. We should either let the mouse event bubble up in the DOM or walk up the SModel in our mouse handler implementation ourselves.

[spoenemann]

Same for selection: clicking on the labels of a core does not select that core.