Expose choice of distance unit for elevation profiles

[nyalldawson]

This PR adds choices of distance units for elevation profiles. It's exposed both for the interactive elevation profiles (via the settings toolbar button) and for layout elevation profiles.

Users can pick to override the default (ie canvas map units) distance unit.

In layout profiles, there's also an option which controls where the distance unit suffixes should be placed. Options are to hide the suffixes, for all values, for the first value, last value or first and last values.

note it is currently NOT possible to expose unit selection for the vertical axis, as this is currently dimension-less and is blocked by missing vertical CRS handling in QGIS. See qgis/QGIS-Enhancement-Proposals#267

[qgis-bot]

@nyalldawson

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[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
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[wonder-sk]

Looking good, thanks!

One thought: how about adding a space between numbers and distance units - "100 m" instead of "100m" ? It seems that's the correct way of writing according to SI - https://grammarhow.com/do-you-put-a-space-between-number-and-unit/

[nyalldawson]

@wonder-sk

how about adding a space between numbers and distance units

Done!

[github-actions]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#8296
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!