This is mainly useful for scripts that generate/load datablocks for their own use and don't want to burry the user under an avalanche of datablocks he/she doesn't care about.
This adds a user pref "Hide .data" which now acts as a default value when opening a new fileselector.
It is also used when creating data select pop menus.
The "ghost" button in a fileselect window is independant from the userpref. It can be turned on/off individually without affecting
Note: When turning the option on/off, it sometimes take a couple of times before the pop menu registers it. Probably some caching thing. Will have to look at it.
Default value is Off.