Check UI files with custom widgets

[Gustry]

It can be nice to check how custom widgets in UI file are imported.

The UI file must contain a block like :

  <customwidget>
   <class>QgsMapLayerComboBox</class>
   <extends>QComboBox</extends>
   <header>qgis.gui</header>
  </customwidget>

It's wrong if the extends contains .h.

CF https://github.com/3liz/lizmap-plugin/blob/master/lizmap/test/test_code_layout.py#L26..L37

[Joonalai]

Than you for the great idea! We will try to implement this soon.

[Joonalai]

Unfortunately, since ui files are not python files, flake8 is not a right tool to lint them. Maybe there should be another tool that can be used as a pre-commit hook to enable this?