Support @Parameter comments in ImageJ 1.x macros

It would be "so cool" (to quote @ctrueden) to have even macros respect the `@Parameter` notation (of course, only for `int`s and `String`s -- and maybe `ImagePlus`ses represented by their IDs).

For objects, it is probably good enough to call toString() when converting them to a macro variable, and then ConversionUtils.convert to migrate strings back to Java object types in the case of outputs. That would be nice and general.