Expose preserve_split, and turn it on

`general.preserve_split` maps straight to the compositor key Patch C adds,
so opening a third window gives three panes in a row instead of quadrants.

Every other key under `general` exists upstream; this one is the fork's
own field on `CosmicCompConfig`. If a rebase ever loses that field the
compositor would ignore the key without saying anything, so the target is
pinned in a test of its own rather than left to the generic registry
checks.

config/cosmic.conf

@@ -45,6 +45,7 @@ $gap = 4
 
 general {
     autotile = true
+    preserve_split = true
     gaps_in  = $gap
     gaps_out = $gap * 2
 }