Finish code to run morphological segmentation from a macro.

Important remark: the macro call will fail if done right after calling the plugin. It needs to wait for the GUI to be constructed.
Add test file to debug Morphological Segmentation plugin.
Add javadoc comments.
Create method to toggle overlay from macro.

Move original toggleOverlay inside CustomWindow where makes more sense.
Add method to show results from macro.

Move showResult method into CustomWindow.
Open grains.tif file by default to speed up debugging.