Show image actual size & hide window

[sportflier]

This utility would be useful to me only if I know the image is being displayed actual size. I've made the following changes:

• Add a button "Actual Size" which changes the size of MainWindow to the dimensions of the source image file.
• Controls dependent on an image having been loaded are now enabled only after an image has been loaded.
• Address issue #5 by adding a 'hide' button
• Minor visual and code cleanup.