niri: add window-rule management

- settings UI for creating, editing, deleting window ruels
- IPC to create a window rule for the currently focused toplevel

fixes #1292

core/internal/windowrules/providers/providers_test.go

@@ -0,0 +1,22 @@
+package providers
+
+import "github.com/AvengeMedia/DankMaterialShell/core/internal/windowrules"
+
+func newTestWindowRule(id, name, appID string) windowrules.WindowRule {
+	return windowrules.WindowRule{
+		ID:      id,
+		Name:    name,
+		Enabled: true,
+		MatchCriteria: windowrules.MatchCriteria{
+			AppID: appID,
+		},
+	}
+}
+
+func boolPtr(b bool) *bool {
+	return &b
+}
+
+func floatPtr(f float64) *float64 {
+	return &f
+}