Add onAnyEvent callback (#3244)

Implement a radical approach to improving abilities of plugins to detect
and handle various changes of micro's state: add onAnyEvent callback
which is called, literally, after any event. A plugin can use this
callback to compare a state after the previous event and after the
current event, and thus is able to catch various events that cannot be
detected using other callbacks.

Some examples of such events:

- change of current working directory
- switching cursor focus between a bufpane and the command bar
- change of message text in the status bar

runtime/help/plugins.md

@@ -72,6 +72,10 @@ which micro defines:
 
 * `preRune(bufpane, rune)`: runs before the composed rune will be inserted
 
+* `onAnyEvent()`: runs when literally anything happens. It is useful for
+   detecting various changes of micro's state that cannot be detected
+   using other callbacks.
+
 For example a function which is run every time the user saves the buffer
 would be: