Fix all warnings from the cppcoreguidelines-pro-type-member-init clang-tidy check (#4426)

src/messages/MessageElement.hpp

@@ -209,7 +209,7 @@ private:
     Link link_;
     QString tooltip_;
     ImagePtr thumbnail_;
-    ThumbnailType thumbnailType_;
+    ThumbnailType thumbnailType_{};
     MessageElementFlags flags_;
 };
 

src/messages/layouts/MessageLayoutContainer.hpp

@@ -90,10 +90,10 @@ struct MessageLayoutContainer {
 
 private:
     struct Line {
-        int startIndex;
-        int endIndex;
-        int startCharIndex;
-        int endCharIndex;
+        int startIndex{};
+        int endIndex{};
+        int startCharIndex{};
+        int endCharIndex{};
         QRect rect;
     };