Add a pixel scale bar

[arjunrajlab]

It would be nice to have another scale bar that shows the number of pixels. I created a branch with the desired look:

https://github.com/Kitware/UPennContrast/tree/pixel-scale-bar

Here's what it should look like:

I made a very small edit to the code where I just add the widget and don't even track it, but probably needs to keep track of the widget and allow for deletion and so on. I think the interface for controlling it should be the following:

1. Put an option here called "Show pixel scalebar" (enabled by default):
2. If you click on either scalebar, brings up this dialog:

   Add a toggle option to the dialog for "Show pixel scalebar".

[arjunrajlab]

Can probably also fix this issue at the same time: https://github.com/Kitware/UPennContrast/issues/669

[arjunrajlab]

Also, depending on how much work it is, could we put in a color picker for the scale bar color?