Draft: Introduce wlr_configurable

shimarin $ ~/opt/wlroots grep "// First find the ack'ed configure" types -R | wc -l 
4

https://en.wikipedia.org/wiki/Rule_of_three_(computer_programming)

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Closes #3206

This MR introduces wlr_configurable, a simple abstraction used to keep track of sent and received configures. Configurable objects don't receive a configure event immediately; instead, the event is delayed, allowing for protocol extensions to send their own configures alongside the way in any order. A wlr_configure which can store multiple object-specific configures tied to the same configure serial is used to achieve that.

TODO: add docs

Breaking changes

• wlr_xdg_surface_configure has been removed.
• wlr_xdg_surface.events.{ack_,}configure now have wlr_configure instead of wlr_xdg_surface_configure as their argument.