bug fix: unloaders can be added after unload has occured

[erikvold]

Came across this while writing a alternate patch to #559

unloaders can be added after unload has occured, which should not be allowed, and can occur under race conditions like using watchWindows on a opened window that has not yet loaded and disabling the addon before the window completes loading

[erikvold]

One issue is that unload can be triggered multiple times and not just on shutdown.

This could be accomplished by making an array of funcs returned from unload, and calling the funcs in that array. We could also add a undo function which unloads everything currently in the queue, without wiping the queue away.

[erikvold]

There is some reason for the race condition in #559 while using listen though, and that may be solvable by some means, but I think a unload function that will immediately call if the unload has already occurred is a good idea regardless.

[Mardak]

Some add-ons explicitly call unload() in response to certain user actions that don't include the add-on being disabled such as toolbar customization opening -- usually followed by a re-init of the add-on. This allows a soft reset without completely disabling the add-on and re-enabling it (and if the add-on page happens to be showing in the Add-ons Manager, it doesn't flash disabled then enabled).