Tooltips don't work on CheckListEditor

Tooltips don't seem to work on `CheckListEditor`s. All other `Item`s I tested, including items editing `List` traits, work fine, but `CheckListEditor`s don't. Tested on Linux and Windows with pyside2.


Made using:
```python
class HasCheckList(HasTraits):

checklist = List(value=["foo"])
checkbutton = Bool(False)

traits_view = View(
Group(
Label("Tooltip not working:"),
Item(
"checklist",
label="Check list editor",
style="custom",
editor=CheckListEditor(values=["foo", "bar", "baz"]),
tooltip="Can't see this!",
),
Label("For comparison:"),
Item("checklist", label="Default editor", tooltip="Can see this"),
Item("checkbutton", tooltip="Can see this")
)
)

HasCheckList().configure_traits()
```