Clean up signal handling

[theoreticalbts]

This PR is a fairly self explanatory refactoring. Here are the main features:

• Rename signals and related methods to be descriptive and consistent
• Always use an XXX_notification struct as the signal argument (simplifies code changes needed to send more data to a signal handler)
• Add pre-apply signal for blocks
• Always use groups (allows control of processing order when needed, so some plugins' handlers can depend on the results of other handlers)
• Remove unused handler (its functionality is now available by checking whether or not you're currently processing a block)

[mvandeberg]

Closes #2230

[mvandeberg]

This was much needed. LGTM