Support the primary clipboard buffer on Unix

[YohDeadfall]

Is your feature request related to a problem? Please describe.

Unix systems have a few types of clipboard buffers: secondary, primary and clipboard. The last one is already supported by Avalonia since it's supported by all platforms. The primary buffer is filled with data on selection and can be pasted through the middle mouse button. That feature is widely supported, so it may be worth to implement it.

Describe the solution you'd like

It requires extending the existing abstraction of the clipboard by providing an enum argument which defaults to the clipboard. If the mode is not supported then null is returned. In addition to that there should be a mouse gesture.

Describe alternatives you've considered

Keep it as an external package.

[maxkatz6]

Should be considered as part of https://github.com/AvaloniaUI/Avalonia/issues/11011

[kekekeks]

Note, that it has to set the clipboard text on every single text selection change. So we'll need to change the way we interact with the clipboard in our TextBox to avoid allocations.