Spawn with a custom window rule

[YaLTeR]

Add a custom window rule argument to niri msg action spawn that applies properties to the first window created by the spawned process. For example, to spawn just one application window as floating, or at specific coordinates.

niri msg action spawn --rule 'open-floating true; default-floating-position x=100 y=100' -- foot

One question is how to represent this in the IPC structures. It would make sense to accept the same KDL as in the config for niri msg action spawn. However, it would also make sense to write out the window rule properties inside IPC structures, and pass them along with the rest of the action as JSON. And on the contrary, I don't like the idea of having a rule: String in IPC which is then parsed as KDL inside the niri server process.

So maybe we could put the window rule properties into the IPC structures and parse the config KDL in the client niri msg process? This unfortunately results in some duplication of window rule structures between the config and the IPC, but I guess that's not too bad.