Convert CreateConnectedInputProxy into full class

Thereby, add more methods that can be reasonably called upon a newly created input slot (even though, technically such input isn't created yet):
* ```operator=``` for constant values
* ```connect```

Additionally, the destructor of the class should emit a warning if the returned ```CreateConnectedInputProxy``` object isn't used properly and as effect, no actual input is created.
Example code that should result in a visible error to the programmer (at run-time):
```cpp
stage->createInput("value1"); // > error: no input is created due to unknown type information
stage->input("value1") << value1; // error: input was never created so could not be found
```

Start by locating CreateConnectedInputProxy and its createInput and input call sites, then read how the proxy currently represents a not-yet-created input. The work is done when constant assignment and connect are supported and discarding the proxy emits a runtime warning, with both examples producing the described diagnostics.

Written by the indexing model from the issue text.