Customer columns: Last Order and Order Count

[Sablednah]

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Hi using 0.9.0. and would love to add 2 columns to the Customers grid to view (and importantly sort by) number of orders and last order placed.

Reading "issues" here I think I need to add to customgrid.xml, but I can't work out what to add? In fact my customgrid.xml (as shipped) already has 1420 lines? Do we merge new XML into the existing file? Confused. I've spend about 4 solid days trying to do this with observers before finding your code. I could add columns either with joins or subselects - but they where not sortable :/

Hope you can help!