app: Add option in the windows menu to hide the image tab bar.

This patch adds a new option located in the windows menu to hide the image tabs bar.

When working with several images at the same time, I always have the Images dialog docked, and use that to switch between images in single window mode. Hiding the tab bar leaves more space in the image window to work.

It's also cool to have the option to hide it if you are working with only one image, since in that case the bar serves little purpose.