Add open-consume-into-column window rule

[y0usaf]

Adds a new initial-configure window rule open-consume-into-column that automatically merges a newly opened window into an existing column containing another window with the same rule, rather than creating a new column.

Motivated by longstanding requests:

• Allow spawning windows in the same column/tab stack #1419: "opening a new app/window scrolls my entire screen which I find very disruptive. I normally immediately consume-or-expel-window-left the spawned window, but this feels tedious to do every single time." — rizo
• Window rule to auto-consume child windows into the same column #410: "all of these windows appear next to the main window in their own column, sometimes pushing the main window partly off-screen... Essentially performing an automatic consume-window-into-column when the child window is opened." — Nairou
• Automatic tab grouping #1125: "I always want my many browser windows when launched in the same column. Also solve the 'open-new-window' problem." — clotodex

window-rule {
    match app-id="^foot$"
    default-column-display "tabbed"
    open-consume-into-column "active"
}

Two strategies:

• "active" — prefer the column to the left of the new window (the previously active column in the common case), falling back to the first match
• "first" — always consume into the first (leftmost) matching column

Only applies to tiled windows. Floating windows are unaffected.