Win32: Prevent going 'up' a directory when already at root.

Related issues: #1810, #1803
Win32: Don't free the result of GFileGetHomeDocumentsDir

Related issues: #1810
Win32: Convert backslashes to slashes in the file open dialogue

Related issue #1803
Don't show modal error when opening a folder as a font fails
Win32: Another case of backslashes to slashes.

This fixes most of the *crashes* in #1803, but not necessarily
what may be considered as 'logic' bugs.
Alter the file open method to work more sanely.

* If a file open action fails (could not load path as a font), then a best-effort
  attempt is made to return the user to the last directory they were at, otherwise
  they will be returned to the default folder.
* If a folder is 'opened' but is not actually a font, then the file opener will
  be redisplayed starting at the contents of that folder.
Win32: Allow browsing to a folder using forward slashes.

* If a user has a trailing forward slash, make the file chooser navigate to that
  path instead of trying to open it as a font.
Win32: Remove obsoleted code from _FVMenuOpen

Now that GFileDirName always returns a path with a trailing slash, this code
here is not needed.
Add option to GFileDirName to always treat as file (for UFO/sfdir)
Merge pull request #1814 from jtanx/fileopen

Various file picker fixes