Implement the OSM down-loader widget

[mvmaltitz]

We need a widget that contains these elements:

A radio button with an option to either input the data manually or download from OSM

See also #267 for the back-end function

[timlinux]

Update: As discussed, for this widget, we need only to implement it like this.

Essentially the widget is just a descriptive label.

The layer data passed to the widget should be returned as-is