Plugins - Inform Plugin Dependencies of Parent Unloading

[TheBrambleShark]

Currently, if a plugin is unloaded, its dependencies are not informed. Unexpected or undocumented behavior may occur when attempting to interact with the unloaded plugin.

When a plugin unloads itself for whatever reason, all dependent plugins should be immediately informed so they may handle the situation as is appropriate.

[Naamloos]

Hello hacktoberfest people, this should be a fairly simple one to tackle!

[Naamloos]

Added priority: low label, as the plugin framework has somewhat low priority at this moment. This will change in the future!