Add a GSS writer feature

## Description
In an application, we probably will not just be reading data, but also writing it. There are workarounds to programmatically create GSS files, but they may not be very easy to implement and error prone. Therefore it would be a good feature to create a writer included in the GSS library which is able to create GSS files easily and programatically.

## Example
```c++
#include
#include

int main()
{
GSS::Writer out("properties.gss");
out.insertClass("button");
out["button"].insertProperty("Height",50);
out["button"].insertProperty("Color","#ff0000");
out.save(); // This may be necessary or not depending in the inplementation.
}
```

The issue names GSS/Writer.hpp and the GSS::Writer API, but no existing implementation files or tests. Start by reviewing the current GSS library and its reader or parser interfaces to understand the file format. Done means a writer can support the shown class and property operations and create a valid GSS file, including save behavior.

Written by the indexing model from the issue text.