Rename Plugin class

[gedankenexperimenter]

Instead of the overly-generic Plugin name, I want to call this class EventHandler, and probably rename the existing plugin event handlers, too (e.g. QukeysEventHandler).

In addition, I might rename EventHandlerResult to EventHandlerSignal. I think EventHandlerSignal::proceed is a better indication of how it's used. Maybe EventHandlerMessage or EventHandlerMsg is better?

[gedankenexperimenter]

Upon further consideration, I want to add LED modes to this refactoring party. I'm not sure that I want to continue using the word "background" in the name, but that's just one detail. The main thing is that, in the restructuring, a "plugin" will become a term for a library that can include the following types:

• EventHandler
• LedMode (or LedBackgroundMode)
• LedModeUpdater (within LedMode)

[gedankenexperimenter]

The renaming has been done, but some files need renaming now.