"New Font" Dialog should perhaps switch file type when the entered filename contains a different (but valid) extension

[justvanrossum]

Scenario:

• I click "New Font..."
• I type "mytestfont.fontra" as the file name
• The file type popup stays at "Designspace", and the resulting project is "mytestfont.fontra.designspace" :)

Not sure if this is something that Qt can do (switch filetype based on live user input)