Removes the `ProcessorMode` enum and the associated `mode` field
from the `Processor` class. Processors now implement three distinct
methods, which they can make use of.

* `on_begin_save`
* `on_end_save`
* `on_key_release`

These all get triggered at the appropriate moments and allow a
processor to perform whatever action it needs to. Processors are
no longer limited to a single operation.

Signed-off-by: Jim Teeuwen <jimteeuwen@gmail.com>