source/base: Make it possible to get the prepare callback

source/base: Make it possible to get the underlying sd_event_source

source/base: Only disable the event if it exists

This shouldn't happen in practice but will be needed to supporting
moving.

source/base: Take an event directly instead of sdevent

Also expose a mechanism for retrieving our internal event.

source/base: Add the prepare callback functionality

source/base: Expose trivial built in methods

Large refactoring