Unify cancel state

We have different wording for the cancel states: cancel, canceled and cancelled.
I think we should unify it for the label (and reports) but also for the internal state (otherwise diversity will keep spreading).
The changing of internal state requires a migration and updating code depending.

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I think we should unify all the wording to "Cancelled"

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I think we should have opinion of native speaker like @dave.

The state should probably be called 'cancelled' where the transition function (verb) is 'cancel()'

I would stick to international english instead of Americanisms.
https://web.archive.org/web/20140307011713/http://www.commissionedwriting.com/THE%20CAMBRIDGE%20GUIDE%20TO%20ENGLISH%20USAGE%20PAM%20PETERS.pdf

I agree with msg54533, I think ideally the state should be ('cancelled', "Cancelled"), and the function or button that puts it into this state should be called 'cancel()' and "Cancel" respectively.

assigned to @dave

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New review258811002 at https://codereview.tryton.org/258811002/#ps280931002

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review258811002 updated at https://codereview.tryton.org/258811002/#ps274941015

To make reviewing these changes easier I am splitting this up into smaller codereviews.

New review317461003 at https://codereview.tryton.org/317461003/#ps327291002

New review309681006 at https://codereview.tryton.org/309681006/#ps321391002

New review325441002 at https://codereview.tryton.org/325441002/#ps311611002

New review321401002 at https://codereview.tryton.org/321401002/#ps297841002

New review323371002 at https://codereview.tryton.org/323371002/#ps311611003

New review291801003 at https://codereview.tryton.org/291801003/#ps321391003

New review301701002 at https://codereview.tryton.org/301701002/#ps321411002

New review325451002 at https://codereview.tryton.org/325451002/#ps303661002