preserve MDAL mesh metadata when editing mesh layers

[uclaros]

Description

When editing mesh layers, all MDAL mesh metadata are dropped from the modified file. This can break formats that rely on that metadata, like the mike21 format.

This PR makes sure the MDAL mesh metadata are read when loading the mesh and stored back to the output file when editing is finished.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 7ce7d5e)

[nyalldawson]

Can you add a test for this?

[uclaros]

Can you add a test for this?

At the moment the MDAL mesh metadata are only used by the ply format which is not good for preserving them perfectly (eg comments can be renamed, leading/trailing whitespaces may be ignored etc)

There is a pending PR that adds metadata handling to mike21 format, I can add a test when this change lands into QGIS.

[nyalldawson]

Is the clang tidy warning correct here?

[uclaros]

Is the clang tidy warning correct here?

This warning is about existing code and not related to this PR.

There seem to be calls to virtual methods during construction, but the proper method is called on runtime. Is that because the base class is abstract and has no implementation for them?

[nyalldawson]

@uclaros I'd add a "final" annotation to the virtual method override -- that should fix the warning