make menupopups with many menuitems scrollable

The attachments show 2 cases where a menupopup with many items leads to inaccessible menuitems because they are rendered off screen. Scrolling them into view will hide the popup (even if ZK-4130) is fixed.

So a usability enhancement could be to add a scrollbar to menupopups in general in case their height will exceed the screen height.

Since those examples are edge cases a scrollbar seems an acceptable fallback to otherwise inaccessible menuitems - and it doesn't affect the behavior of menupopups with less menuitems.

Workaround

a simple style can add such a scrollbar on demand (commented out in the examples) making the menu items accessible again, even in nested levels

	.z-menupopup {
		max-height: 100%;
		overflow-y: auto;
	}