DefaultDatasetService: remove semi-colons from RU

The ReflectedUniverse.exec methods can not accept trailing semi-colons
in executed commands.

Fix suggested on https://github.com/imagej/imagej-common/issues/34
by user 9035768.
Work around RU array, nested class limits

ReflectedUniverse has the following limitations:
* vararg parameters only accept arrays
* can't construct arrays
* can't import nested classes with a .
* since .'s can't be used, the enclosing classname has to be used to
  reference a nested class

All four of these limitations were affecting the setting of the ImgMode
in DefaultDatasetOpener#open.

These issues have now been resolved by use of java.lang.reflect.Array
and properly structured import statements.