...with known plugins

Add new plug-in file-raw-placeholder.c that registers itself for
loading all RAW formats from file-raw/file-formats.h, but does nothing
except returning an error message pointing to darktable and
RawTherapee.

When no real RAW loader is installed, this plug-in is selected
automatically as RAW loader, otherwise the first installed RAW loader
is used. Selecting another in prefs still works as before.