Disable file sniffing in MIME detection on Windows.

glib on Windows doesn't do anything particularly useful with the sniffed data,
and opening files on Windows can be *really* slow (especially if it's an
executable/.exe).
GScrollBar: greatly reduce min-thumbsize clamping threshold
GScrollBar: compute a size offset when clamping the thumb size
Merge pull request #1707 from jtanx/win-mime

Disable file sniffing in MIME detection on Windows.
Merge pull request #1646 from adrientetar/scrollbars

Fix gdraw scrollbars