Notifications: replace newlines in label-copy because of const char* title

The variable `title` is defined as `const char*`, which means, that
`strchr()` returns a `const char*` as well according to
https://www.cplusplus.com/reference/cstring/strchr/

But in the same line the return value is assigned to a non-const
`char*`, which shouldn't be allowed (error with `-pedantic`).

Because the `lv_label` creates an internal copy of the title sting, just
modify that one instead and replace newline in the copied string.

src/displayapp/screens/Notifications.h

@@ -62,10 +62,6 @@ namespace Pinetime {
         };
 
       private:
-        struct NotificationData {
-          const char* title;
-          const char* text;
-        };
         Pinetime::Controllers::NotificationManager& notificationManager;
         Pinetime::Controllers::AlertNotificationService& alertNotificationService;
         Pinetime::Controllers::MotorController& motorController;