TeamWindow: Fix accidental double message sending.

- When we explicitly handle sending B_COPY/B_SELECT_ALL, don't
  then pass control back to BWindow::DispatchMessage(), as that will
  wind up sending the message to the target view twice.
Debugger: Add support for copying variable values.

- Implements a simple copy option in the variables context menu that
  allows one to copy the displayed value to the clipboard.