layout: rethonk layouts from the ground up (#12890)

Rewrites layouts to be much smaller, and deal with much less annoying
BS. Improves the overall architecture, unifies handling of pseudotiling,
and various other improvements.

src/layout/algorithm/floating/default/DefaultFloatingAlgorithm.hpp

@@ -0,0 +1,26 @@
+#include "../../FloatingAlgorithm.hpp"
+
+namespace Layout {
+    class CAlgorithm;
+}
+
+namespace Layout::Floating {
+    class CDefaultFloatingAlgorithm : public IFloatingAlgorithm {
+      public:
+        CDefaultFloatingAlgorithm()          = default;
+        virtual ~CDefaultFloatingAlgorithm() = default;
+
+        virtual void newTarget(SP<ITarget> target);
+        virtual void movedTarget(SP<ITarget> target, std::optional<Vector2D> focalPoint = std::nullopt);
+        virtual void removeTarget(SP<ITarget> target);
+
+        virtual void resizeTarget(const Vector2D& Δ, SP<ITarget> target, eRectCorner corner = CORNER_NONE);
+        virtual void moveTarget(const Vector2D& Δ, SP<ITarget> target);
+
+        virtual void swapTargets(SP<ITarget> a, SP<ITarget> b);
+        virtual void moveTargetInDirection(SP<ITarget> t, Math::eDirection dir, bool silent);
+
+      private:
+        CBox fitBoxInWorkArea(const CBox& box, SP<ITarget> t);
+    };
+};