Add action to user detail dialog to search for their messages in room

In other messaging applications (e.g. Discord) this is possible through
text modifiers like "from:@user". We don't support that, and I'm not
super keen on implementing yet-another-parsing-thing, so an action in
the user detail dialog should work for now.

Very useful to sift through large rooms but when you only care about a
specific person's messages (maybe your own?)

src/timeline/models/searchmodel.h

@@ -37,6 +37,11 @@ class SearchModel : public MessageModel
      */
     Q_PROPERTY(bool searching READ searching NOTIFY searchingChanged)
 
+    /**
+     * @brief If set, limits the search to this specific sender.
+     */
+    Q_PROPERTY(QString senderId READ senderId WRITE setSenderId NOTIFY senderIdChanged)
+
 public:
     explicit SearchModel(QObject *parent = nullptr);
 
@@ -45,6 +50,9 @@ public:
 
     bool searching() const;
 
+    QString senderId() const;
+    void setSenderId(const QString &sender);
+
     /**
      * @brief Number of rows in the model.
      *
@@ -61,6 +69,7 @@ Q_SIGNALS:
     void searchTextChanged();
     void roomChanged();
     void searchingChanged();
+    void senderIdChanged();
 
 private:
     std::optional<std::reference_wrapper<const Quotient::RoomEvent>> getEventForIndex(QModelIndex index) const override;
@@ -71,4 +80,5 @@ private:
     std::optional<Quotient::SearchJob::ResultRoomEvents> m_result = std::nullopt;
     Quotient::SearchJob *m_job = nullptr;
     bool m_searching = false;
+    QString m_senderId;
 };