Fix cv export templates

It is bad practice to write code like this:

```
template <typename T> class __declspec(dllexport) TemplatedClass
{
...
}
```

If we want to export something, then we should add explicit exporting of templated class specializations. For example:

```
template class __declspec(dllexport) TemplatedClass<float>;
```

This fix removes CV_EXPORT from templated classes only, no specializations are added. Any suggestions or bug reports are welcome.