Initialize Window by observer instead of by plugin fields

[janhohenheim]

Objective

• Part of Split the Window component for more granular change detection #19644
• Window is biiiiig, so we want to split it
• However, to split it, we need a blessed pattern for how to initialize all these new components
• The old way was to have a field on WindowPlugin
• But if we split Window into 10 components, we'd have 10 new optional initializer fields on WindowPlugin
• That's just a worse version of required components

Solution

• Use required components to build up the Window instead
• Tell users through the examples to use an observer instead of the plugin field
• While I was on it anyways, I removed the window titles of many examples, as even pre-PR they were mostly boilerplate that didn't add much didactic value

Testing

• TODO: need to still test this some more

[janhohenheim]

Blocked by #20019