Dialog window for widget promotion needs usability improvement in regards to PySide

PySide has been an official part of Qt for several years now. Current version of Qt Creator even offers the option of working on a standalone Qt Python project as seen in the New Project Wizard.

This suggestion is aiming to improve the usability of the Promote to dialog window in Qt Designer by introducing a minor change to the text in the Header file label as well as the header of the column with the same name in the table Promoted Classes. I propose for it to become Header file/Python module.

While Qt Creator (and Designer) has come along quite a bit lately in regards to the official support of PySide it still needs more work. Having to enter a Header file for a UI form that is used in a Python project is very confusing especially to non-C++ people. It took me some  time by searching online and analyzing the generated Python module (using `pyside2-uic`) to figure out that the content of the line edit for the header file is a direct copy-paste for the name of the module, where the promotion class is coming from (had I done this before in C++ it would have probably been more obvious).

I created a QWidget, which I then promoted to a QVideoWidget (which is missing as a widget from the palette of available components that can be dragged and dropped onto the form in Qt Designer). Since the UI file will be used for a Python module using PySide2 I found out that I had to put PySide2.QtMultimediaWidgets so that the generator created the entry from PySide2.QtMultimediaWidgets import QVideoWidget. *Here the notation that a *header file needs to be entered into the field is confusing since it considers only the C++ scenario.