Let plugins choose the priority order of their events

[oprypin]

Add a decorator that sets a priority value for plugins' events.
Events with higher priority are called first.
Events without a chosen priority get a default of 0.
Events that have the same priority are ordered as they appear in the config.

Recommended priority values:
100 "first", 50 "early", 0 "default", -50 "late", -100 "last".
As different plugins discover more precise relations to each other, the values should be further tweaked.

@plugins.event_priority(-100)  # Wishing to run this after all other plugins' `on_files` events.
def on_files(self, files, config, **kwargs):
    ...