This disables the "Automatically" currency listbox entry in the number
formats dialog and makes it unselectable, unless such currency format
was already used at the current position of a loaded document.

Getting completely rid of the listbox entry if not used is more
cumbersome as the positions are tied to the list entries in
SvxNumberFormatShell::aCurCurrencyList, but at least the disabled entry
prevents accidental or ill intended selection of that awkward format.

Change-Id: I16f3e88feede34e3f83afb108f253fb53d82d34d