Add active-window-* as options for floating window relative-to rule

[contagnas]

This adds "active-window-*" options to the RelativeTo enum for default-floating-position. These position the new floating window relative to the currently active window. My use case to emulating window parenting for applications` that don't support it, but I could imagine other uses as well.

Demo:

2026-02-16_11-55-53.mp4

If there is no active window the new window goes to the center of the screen.

Considered some alternative approaches, because IMO this enum is a little bloated after this change:

• new option in window-rule like default-floating-position-relative-to-active-window that would accept the same relative-to options
• hook to an external program that would decide coordinates

but just adding these to the enum seems more straightforward and understandable.

[contagnas]

somewhat related: #978