window: Remember relative position after constraining with custom rule

In order to allow a window with a custom rule placement to be moved together with its parent, the final rule used derived from the constraining were used for subsequent constraints. This was not enough as some constraining cannot be translated into a rule, such as sliding across some axis.

Instead, make it a bit simpler and just remember the position relative to the parent window, and use that the next time.

Fixes: #332 (closed)

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