Hide focus outline on overlay items when using pointer input

[sissbruecker]

Currently some components show a focus outline on an item when opening their overlay using pointer input:

• vaadin-select - Shows focus outline on first or selected item
• vaadin-context-menu - Shows focus outline on first menu item

Compared to that other components do not show a focus outline when opening their overlay: vaadin-combo-box, vaadin-menu-bar

We briefly discussed this when implementing the pointer focus ring and concluded that a focus outline around an item should only be shown when using keyboard navigation, but not with pointer input.

One open point was what should happen with the focus itself. It should probably still move into the overlay, on some item, to allow switching from mouse to keyboard for example.

Another thing to note is that if the pointer focus ring is enabled, then we might still want to show a focus outline around the vaadin-select button, as otherwise no focus outline would be visible anymore. Currently the pointer focus ring is not implemented for vaadin-select. See #2904 for more info about that feature.