(e.g. parameters) when they are run in the "edit in-place" mode

This allows algorithms to dynamically adapt their behavior to make them compatible
with in-place mode. Previously, some useful algorithms could not be
run in-place because they alter a layer's structure (e.g. adding new
fields).

Now, these algorithms have a means to detect that they are being
run in-place and change their input parameters accordingly. E.g.
an algorithm which usually adds new fields to store calculated
values (such as "add xy fields to layer") could instead expose
field parameter choices to ask the user to pick from existing
fields in which to store the calculated values, thereby avoiding
the need to change the table structure and making them eligable
for running in-place mode.

Note that this needs to be handled algorithm-by-algorithm, it's
not automatic! It's just the raw api to allow this...