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
 }

cosmic-conf/src/schema.rs

@@ -161,6 +161,17 @@ pub const REGISTRY: &[Entry] = &[
         validate: None,
         doc: "Automatically tile new windows",
     },
+    Entry {
+        conf: "general.preserve_split",
+        targets: &[Target::Direct {
+            component: COMP,
+            version: 1,
+            key: "preserve_split",
+        }],
+        ty: Ty::Bool,
+        validate: None,
+        doc: "Open new windows alongside the focused one instead of splitting it",
+    },
     Entry {
         conf: "general.active_hint",
         targets: &[Target::Direct {
@@ -359,6 +370,18 @@ mod tests {
         assert_eq!(e.targets[0].component(), "com.system76.CosmicComp");
     }
 
+    #[test]
+    fn preserve_split_targets_the_forks_own_comp_key() {
+        // Unlike every other `general` key this one does not exist upstream --
+        // it is a field this fork adds to `CosmicCompConfig`. If a rebase ever
+        // drops that field the compositor silently ignores the key, so pin the
+        // exact target here rather than trusting the generic registry checks.
+        let e = lookup("general.preserve_split").unwrap();
+        assert_eq!(e.targets.len(), 1);
+        assert_eq!(e.targets[0].component(), "com.system76.CosmicComp");
+        assert!(matches!(e.ty, Ty::Bool));
+    }
+
     #[test]
     fn every_entry_has_at_least_one_target() {
         for e in REGISTRY {