Add basic cross-signing support

(cherry picked from commit e1795076c8c41a34b91c31df35d1e344d0b14887)

src/controller.h

@@ -51,6 +51,7 @@ class Controller : public QObject
     Q_PROPERTY(QStringList accountsLoading MEMBER m_accountsLoading NOTIFY accountsLoadingChanged)
 
     Q_PROPERTY(bool ssssSupported READ ssssSupported CONSTANT)
+    Q_PROPERTY(bool csSupported READ csSupported CONSTANT)
 
 public:
     static Controller &instance();
@@ -97,6 +98,7 @@ public:
     Q_INVOKABLE void removeConnection(const QString &userId);
 
     bool ssssSupported() const;
+    bool csSupported() const;
 
 private:
     explicit Controller(QObject *parent = nullptr);