search with Component.with_uid()

[niccokunzmann]

Since #658 and #657, UID is quite important to reference other components.

I would like to retrieve all subcomponents with a particular UID!

component = icalendar.Component()
components = component.with_uid("uid")

Requirements:

• the return type is a list of components, including alarms
• they all have the uid
• use walk() to generate the result
• create a nice example
• Document this not just in the API but in a how-to "Find all components with a certain UID"