Editor __init__ arguments should be reviewed

There are a number of issues with the arguments that are passed to the `Editor.__init__` method.

- the `parent` argument is never used (instead it is passed to the `prepare` method later)
- the `tooltip` argument probably shouldn't be _required_
- the `item` possibly _should_ be passed (except it isn't available in all cases where an editor is created, eg. by a `TableEditor`)
- the `item` is currently only used for button editors (to get the button label if one isn't supplied) and by the wx editors to determine sizing behaviour
- we should probably require as few positional arguments as possible and move to keyword-only

All-in-all this probably needs a redesign, but very carefully, as we are talking public parts of the APIs.