Save dialog silently replaces file extension

[Patrick Storz]

Inkscape automagically replaces a potential file extension in the given file name to match whatever format is selected in the dropdown.

Steps to reproduce:
- Select PostScript file format (".ps")
- Set file name to "drawing.svg"
- Save the file
Result:
- The file is saved as "drawing.ps"

This affects both, the GTK save dialog as well as the native win32 save dialog.

While this behavior is probably desirable in most cases it could be unexpected for some users (maybe they specifically want multiple file extensions or even hope Inkscape would possess a functionality to guess the filename from the typed file extension like some other software does)

I'm not sure what the best solution would be. Three options I can think of:
- Warn the user (i.e. "the file extension you typed
  does not match the file extension you chose) and
  prevent them from saving.
- Warn the user that the file extension dies not
  match and will be fixed automatically
- Do not mess with the file name the user typed at all
  and simply append the proper extension (e.g. resulting
  in "drawing.svg.ps" in the example above).