SVG font input empty chars not output to SFD or fonts

Magic flag widthset was not being set for input glyphs upon parsing
from SVG font files.  This flag is used by sfd.c routine SFDOmit()
to judge whether a character was significant enough to be output.

Upon first input from SVG font file, empty glyphs (not containing any
drawing instructions / spline info) would appear in character view
(as X'd boxes).  But these would not be output to a new SFD file or
any generated fonts.  Restarting FF using the previous SFD file as
input would find none of the empty characters - they'd disappeared.

This would affect such glyphs as '.null', 'space', and maybe
'nomarkingreturn'.

Fix adds one line to set flag into svg.c routine SVGParseGlyphArgs().
Make u_GFileIsDir work
Fix file chooser if given a folder path with no trailing backslash
Fix 'Open' command from the menus on Windows. Fixes #1466

The current font is required to determine the last used directory to be shown.
The method used is the only reliable way of getting hold of that last used font.
Free the return value of GFileDirname
In GIOGetMimeType(), removed sniff_data, add comments (improvements).

Always sniff file data for mime type
Makes no much sense otherwise.
Credit = 2012-10-24 18:05:13 - Khaled Hosny

Add GIOGetMimeType() comment.
Credit = 2012-10-29 21:10:00 - Khaled Hosny
Fix bugs in GIOGetMimeType()

Converted sniff_length into a define because this is a compiled binary.
g_content_type_guess() was wasted compute time at the front, and the
result was thrown away, so moved it to the back as a last resort if
sniffing data for mime type failed.
Accept the possibility of "content_type==NULL" after all these check.
Merge pull request #1480 from tshinnic/sfd_empty_char

SVG font input empty chars not output to SFD or fonts
Merge pull request #1484 from JoesCat/master

More GIOGetMimeType() fixes
Merge pull request #1482 from jtanx/menuopen

Menuopen