fix(framework): component children that are slots now invalidate parent upon slotchange

[vladitasev]

There are composition scenarios where a component, say ui5-upload-collection creates a ui5-list in its shadow root and slots all its children to the list transitively by just rendering a slot in the list.

Example:

<ui5-list
	mode="{{mode}}"
	@ui5-selectionChange="{{_onSelectionChange}}"
	@ui5-itemDelete="{{_onItemDelete}}"
>
	<slot></slot>
</ui5-list>

The problem with this is that the framework will only invalidate the ui5-list whenever its children are added/removed, but in this case the only child is the slot and it is completely static. Yet, when the user adds items to the ui5-upload-collection, they will be rendered transitively by the list, but the list will not be updated. Therefore some of the children may not be displayed properly.

The reported issue is that when you add items to the upload collection in "Delete" mode, the new items don't get a "Delete" button until the list is invalidated by something else.

Therefore, this change adds some extra capabilities for components with managedSlots: true. If they have <slot> children, these will have slotchange attached/detached automatically by the framework, and the parent component, in this example ui5-list will be invalidated whenever the slotchange event is fired, which is upon adding/removing elements to the slot.

In the future, this may be easily enhanced to work recursively by doing the same for slots inside slots, but for the time being this might not be necessary.

Additionally, a new isSlot utility is added to check if a Node is slot.
Additionally, ui5-input was changed to use the getSlottedNodes function, already provided by UI5Element.js instead of re-implementing it, to easily get the list of all slotted children in the valueStateMessage slot.

This change will affect several components: ui5-upload-collection, ui5-tree, ui5-datepicker, etc... mostly components that transitively slot content to the input or list.

fixes: #1647