Python: Auto-detect and change interpreters

At the moment QtCreator relies on the default python on the system, but the development of python applications rarely happens using that interpreter.

There are many solutions out there for this, but most of them follow this idea:

1. Create a Python environment,
2. Activate python environment,
3. Install packages/modules in the environment,
4. Use the environment's interpreter.

Even though the user can modify the Run Configuration and select the proper interpreter, it could be nice if also a drop-down menu with the available interpreters is displayed.
Also, the "Run" step will need to verify that the environment is active because using only the interpreter path does not guarantee that the modules will be looked up inside the same environment.