The file filter wild card was defined incorrectly causing the file picker
to not display *.stp and *.step file extensions. Change the wildcard per
wxFileDialog documentation.
Prevent the file extension from being forced to .stp even when the user
defined a different extension in the file picker.
Override TransferDataFromWindow to test for file existence and warn user
only once if a file overwrite can occur.
Minor STEP export dialog string and layout improvements.
Make kicad2step honor user's file extension rather than always setting it
to .stp.
Fixes lp:1709636
https://bugs.launchpad.net/kicad/+bug/1709636
Wayne Stambaugh
jean-pierre charras
Simon Wells
Cirilo Bernardo