Always notify watchers of plugin updates

[dcalhoun]

During plugin updates, each plugin.onUnload is invoked regardless of whether the plugin changed. This commit ensures that each plugin.onApp is invoked once all plugins have been updated. Fixes #2415.

[dcalhoun]

@chabou You originally stated:

onUnload method should be called only on removed plugins imo.

I agreed with that originally, but as I thought about it more I'm not sure that would work. For example, if a plugin changes, the plugin's onUnload and onApp need to be invoked so that Hyper ensures that it executes the plugin's latest setup.

In theory, we could change Hyper to only invoke onUnload for removed/changed plugins and only invoke onApp for added/changed plugins, but things quickly increased in complexity. In the end, I felt sticking with calling onUnload and onApp for every plugin was the most straightforward solution.

[dcalhoun]

The PR is ready to be merged. Please let me know if you have any feedback.

[dcalhoun]

@chabou @rauchg @leo @matheuss sorry to bother, but is there anything I can do to help move this PR forward?

[leo]

Thank you!