Autocompletion for role button in relation dialog

[stoecker]

The role field in relation dialog should support autocomplete and the drop-down-box (prefilled with the roles from presets, which are currently not parsed).

Also the dialog should remember the last selection of closing the session. Usually the chance is high that one handles equal roles one after another (e.g. for multipolygons, routes, ...)

@bastiK: Can you do this? You did a lot of autocomplete changes lately.