DatasetIOPlugin: make low priority

The problem seems to be that SCIFIO is too aggressive about what it
thinks it can handle; e.g., when trying to open a .txt file, the
DatasetIOPlugin gets selected.

To avoid this, we simply move DatasetIOPlugin to the bottom of the list.
Although this is a hack which ultimately we should not need, determining
whether an arbitrary file is in *any* supported image format is going to
be an expensive operation no matter what, so for performance reasons it
makes sense to leave the priority of this I/O plugin lower anyway.
LUTService: add a FIXME note about display of LUTs

As things stand, we do not have a proper Display implementation for
LUTs. Simplest would be to enhance the ImageDisplay to support
ColorTables in addition to just Data objects. Then ColorTables would
work as output parameters; e.g., OpenFile could open and display them.
Add I/O plugin stub for scripts

It doesn't work yet, because the ScriptService does not yet provide the
API necessary to actually open the script in the script editor.