Pre-select icon when opening icon dialog

Will select the given icon once the dialog opens, makes it much more convenient to use. Something that has been bothering me for years.

I originally also had code to switch categories if the icon isn't in the current one, however, inode-directory is in Mimetypes, so opening icon dialog for any folder with default icon would switch from the "Places" category to "Mime types", which isn't really what we want. But looking for the icon in the current category covers the 90% use case of toying around with different folder icons or app icons.