Improve the handling of switching link preivews on and off.

First make sure that the global setting is tied into the room setting, previously it was a bit of a patchwork that worked more by luck than judgement. The two levels of global and room level are properly tied together in a hierarchy.

Add a message in the room when global notifcations re turned off. This has caused confusion in the past when people don't realise there are 2 levels.

src/neochatconnection.h

@@ -32,6 +32,11 @@ class NeoChatConnection : public Quotient::Connection
      */
     Q_PROPERTY(QString label READ label WRITE setLabel NOTIFY labelChanged)
 
+    /**
+     * @brief Whether URL previews are enabled globally.
+     */
+    Q_PROPERTY(bool globalUrlPreviewEnabled READ globalUrlPreviewEnabled WRITE setGlobalUrlPreviewEnabled NOTIFY globalUrlPreviewEnabledChanged)
+
     /**
      * @brief The model with the account's 3PIDs.
      */
@@ -182,6 +187,9 @@ public:
     int badgeNotificationCount() const;
     void refreshBadgeNotificationCount();
 
+    bool globalUrlPreviewEnabled();
+    void setGlobalUrlPreviewEnabled(bool newState);
+
     bool directChatInvites() const;
 
     // note: this is intentionally a copied QString because
@@ -196,6 +204,7 @@ public:
     LinkPreviewer *previewerForLink(const QUrl &link);
 
 Q_SIGNALS:
+    void globalUrlPreviewEnabledChanged();
     void labelChanged();
     void identityServerChanged();
     void directChatNotificationsChanged();
@@ -230,6 +239,7 @@ private:
     void connectSignals();
 
     int m_badgeNotificationCount = 0;
+    bool m_globalUrlPreviewEnabled = true;
 
     QCache<QUrl, LinkPreviewer> m_linkPreviewers;