This PR exposes symbols of all classes. This is necessary when subclassing. Since warnings are being thrown on Windows, those are disabled by pragmas.

Here is my understanding on when this is safe:

• It is always safe to call any method of this class, that's why method-wise symbol exporting works without warning.
• When deriving from a class and just USING a type compiled inside the base class (e.g. in the form of a PIMPL or like the SelectionHandlers in markers), this is always safe.
• Private members are safe (but may still throw warnings when they are from std - Microsoft itself says warnings can be ignored at that point, see https://msdn.microsoft.com/library/esew7y1w.aspx).
• When deriving from a class and base class and derived class are compiled on the same system and with the same compiler.

It will only ever be problematic when a client derives from this class.

In our case, this implies it's mostly safe to set pragmas. The only problem I can see happening is when a client derives from e.g. MarkerBase and uses a different version of visualization_msgs::msg::Marker::ConstSharedPtr or rclcpp::Time. But I don't see how this can be done without breaking the ROS installation even earlier. To be sure, I added a comment explaining what's happening if ever somebody encounters a problem.