Tree should allow control of visual indentation spacing

[DavinChurch]

Allow specification of visual indentation spacing for each level of depth.

[johndaintree]

This can be done with []NA (SendMessage TVM_SETINDENT). Is that enough, or do you really need a property?

⎕na 'u user32|SendMessage* U U U U'
SendMessage f.t.Handle(4352+7)100 0

[DavinChurch]

I can't port ⎕NA calls to EWC, so I'm routing everything through the ⎕WC-family of calls to give them an interface to support.