feat(UI5Element): invalidateOnChildChange works for slots

[vladitasev]

Currently, the slot metadata setting invalidateOnChildChange only works for directly slotted UI5 Elements inside the slot.

This change enables the functionality for transitively slotted children too. This is relevant for <ui5-multi-input> which uses internally <ui5-tokenizer> and the <ui5-token>s are transitively slotted from the MultiInput to the Tokenizer, and finally the Tokenizer gets each token wrapped in a <slot> element.

Notes:

• the _getChildChangeListener method's return value is always truthy, it doesn't need to be checked
• unfortunately the slotchange event does not provide information on added/removed nodes (unlike f.e. MutationObserver), therefore a WeakMap is used to store all children per slot between slotchange calls.

related to: https://github.com/SAP/ui5-webcomponents/pull/9005