Support slotted dialog elements

[devpaul]

Add support for slotting in custom dialog elements for Clippy.

• dialog should remain hidden
• when an element is slotted in a slotchange event will fire
• if there is content display the dialog
• if there is no content, remove the dialog

[devpaul]

Resolved with 397d80d3c42a7b7813cda389d7b617a978ad60b2