Keep attached events on overwriting

Attention: This is a breaking change.

This MR changes the way events are overwritten/redefined using mf.unregister_event or mf.overwrite_event.

Description

By default, a call to any of the mentioned methods, all events of the given name are unregistered from miniflask (either only their function cache or any of their future uses), and also all events that are attached to the event to be changed.

A typical used-case, however is the transformation of events using other modules using before_ and after_ event hooks (attached events). With other words, replacing an event should not replace the hooks used to modify or inspect the calls when changing its function.

This MR thus, by default, keeps all attached events upon reregistration, but allows the user to remove all additional (typically) user-events as well.

Things done in this MR

• added keep_attached_events to mf.unregister_event and mf.overwrite_event

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created