See https://github.com/qgis/QGIS-Enhancement-Proposals/issues/86
- Checking/unchecking a group doesn't change the check state of its children.
A node is visible if and only if it is checked and all its parents too.
- There is no more a semi-checked state for a group
- Ctrl-clic on a unchecked group will check the group and all its descendants.
- Ctrl-clic on a unchecked layer will check the lager and all its parents.
- Ctrl-clic on a checked group will uncheck the group and all its descendants.
- Ctrl-clic on a checked layer will uncheck the layer and all its parents.
- Those actions are available in contextual menu items in the tree view.
- Invisible layers because they or their parent(s) is unchecked are greyed out.
