[Enhancement] Make "allow system access" a per-script property

[GoogleCodeExporter]

I don't see much point in the long run to allow/disallow certain functions
(ATM only system access, more may be added in the future) for all scripts
alike.

Instead, I as a user want to assign permissions based on how much trust I
put in a script (and its origin, author, ...), rather than either
 * prevent access for all scripts to be safe, thereby breaking some scripts
 * grant access to all scripts for full functionality, thereby possibly
risking system integrity. In addition, if no state is set, a popup could
ask the user if he wants to "grant access this time", "grant access
always", "prevent access this time", "prevent access always".

Original issue reported on code.google.com by st.loeffler on 26 Apr 2010 at 10:53