refactor: refactor some fields and code to use "colour" spelling (#2058)

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## Description

_A description of the change, what it does, and why it was made. If
relevant (e.g. UI changes), **please also provide
screenshots/recordings**:_

Rename a bunch of code/struct field names to use "colour". Note this
should have no functional change to config settings as I was already
aliasing "colour" for "color" settings, this just flips it internally.

As for why, I'm Canadian, I grew up spelling it this way, sorry.

## Issue

_If applicable, what issue does this address?_

Closes: #<issue-number>

## Testing

_If relevant, please state how this was tested (including steps):_

_If this change affects the program, please also indicate which
platforms were tested:_

- [ ] _Windows_
- [ ] _macOS (specify version below)_
- [x] _Linux (specify distro below)_
- [ ] _Other (specify below)_

## Checklist

_Ensure **all** of these are met:_

- [x] _If this PR adds or changes a dependency, please justify this in
the description_
- [x] _If this is a code change, areas your change affects have been
linted using (`cargo fmt`)_
- [x] _If this is a code change, your changes pass `cargo clippy --all
-- -D warnings`_
- [x] _If this is a code change, new tests were added if relevant_
- [x] _If this is a code change, your changes pass `cargo test`_
- [x] _The change has been tested to work (see above) and doesn't appear
to break other things_
- [x] _Documentation has been updated if needed (`README.md`, help menu,
docs, configs, etc.)_
- [x] _There are no merge conflicts_
- [x] _You have reviewed your changes first_
- [x] _The pull request passes the provided CI pipeline_

## Other

_Anything else that maintainers should know about this PR:_

docs/content/usage/widgets/process.md

@@ -64,11 +64,11 @@ is added together when displayed.
     <img src="../../../assets/screenshots/process/process_grouped.webp" alt="A picture of grouped mode in a process widget."/>
 </figure>
 
-!!! info
+!!! Info
 
     Note that the process state and user columns are disabled in this mode.
 
-!!! info
+!!! Info
 
     Note that if tree mode is also active, processes cannot be grouped together due to the behaviour of the two modes
     somewhat clashing. This also reflects with default modes like `group_processes`.
@@ -103,7 +103,7 @@ A process in tree mode can also be "collapsed", hiding its children and any desc
 ++plus++, or ++left++ keys, or clicking on an entry. It can be expanded by using the ++minus++, ++plus++, or ++right++
 keys, or by clicking on the entry again. The ++space++ key can also be used to toggle between the collapsed and expanded states.
 
-!!! info
+!!! Info
 
     Note that if tree mode is active, processes cannot be grouped together due to the behaviour of the two modes
     somewhat clashing. This also reflects with default modes like `group_processes`.