Need to remove unloader associated to windows when they unload

[erikvold]

This is basically the same issue as mozilla/prospector#9

[erikvold]

@philikon can you review https://github.com/erikvold/BarTabLite/compare/issue-1 when you have a chance please? also please see #2 it is related.

[philikon]

I understand the problem now. I fail to remove the unloader for a particular window when the window is closed. Thanks for the pull request, will have a look.

[erikvold]

Another technique you might be able to use to resolve this is to not use a unloader function. For example, if you had a window listener that added node X, then during shutdown you could enumerate through open windows and remove the node, thus no longer requiring a unloader at all.