Make determining the mime type faster for folders

KFileItem::determineMimeType() not only determines the mime type, but
also the icon. For folders, it looks for a .directory file inside the
folder, where a custom icon might be stored. This can take quite a bit
of time and cause the problem that some folder's type still appears to
be "unknown" when the view is shown.

We can work around this problem by caching the folder mime type in a
static QString and applying to to all folders, which can be identified
easily with KFileItem::isDir(),


FIXED-IN: 4.11.1
REVIEW: 111830