Implement xdg-decoration-unstable-v1 to request server side decorations

[Consolatis]

This allows requesting server side decorations if supported by the compositor. Adds a new config option window.decorations with values

• yes (always request server side decorations)
• no (always request client side decorations, e.g. no decorations)
• auto (tell the compositor to use whatever it prefers)

The new setting defaults to auto.

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I am not sure about the default for the new setting, feel free to request changes.

[ii8]

Seems good, I would have picked the same default. Thank you!