KFileItem optimization

[Jakub Stachowski]

Hello,



After entering large directory but before displaying its contents, Dolphin 
spends some time laying out item in a view 
(QListViewPrivate::doItemsLayout()). Of that time about 42% is spent getting 
icon name for each KFileItem. It is slow operation, because of mime-
>is("application/x-desktop") check, which constructs KMimeType object for 
application/x-desktop type again and again. Attached patch adds fast path to 
KMimeType::is by allowing to ignore resolving aliases. It is not a problem in 
case of KFileItem, because application/x-desktop is not an alias.
This makes KFileItem::iconName() about 4x times faster on first use.
It messes with kdecore so I would like to get second opinion on this patch.

 Another possible optimization: for each item, KFileItemDelegate checks actual 
size of file icon. Is this really necessary? Or maybe it would be enough to 
check size for first icon in the view and then reuse it.
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