Menuopen

This fixes a few minor bugs as well as #1466.

* The *Open* command from any view (Font/Metrics/Bitmap/Char) should now work on Windows
  - How this works: The displayed folder in the *Open Font* dialogue is the folder of the currently active font, or *My Documents* if no font is open.
* u_GFileIsDir now works like GFileIsDir
* Make the file chooser treat directories as directories instead of files if the path has no trailing backslash