PreviewSettingsPage: update JPG preview plugin settings if needed

In KDE <= 4.6, the 'jpegrotatedthumbnail' plugin was used to provide
thumbnails for JPG images which are rotated correctly. In KDE 4.7, this
is done by the usual 'jpegthumbnail' plugin if it is configured
accordingly. The update of the settings is done in KFilePreviewGenerator.
However, if Konqueror is run in web browsing mode and the settings dialog
is opened, KFilePreviewGenerator does not have a chance to do the update.
To make sure that the correct plugins are shown in the settings, this
commit adds a plugin update to PreviewSettingsPage.

The old code that just removed the 'jpegrotatedthumbnail' plugin without
enabling the 'jpegthumbnail' plugin is removed.

CCBUG:277251
FIXED-IN:4.7.0