[4.7.5] Eliminate xxd and w3c dependencies.

[rpatters1]

This pull request updates mnx importexport to use a new version of mnxdom that eliminates the following troublesome dependencies.

• The xxd utility is no longer required at build time.
• The w3c mnx GitHub repo is no longer fetched at build time.

The change in mnxdom is a narrow patch to version 3.0 (called 3.0.1) that refactors the build but leaves the rest of the code unchanged. PR #33958 incorporates this cmake change for the main branch along with many updates to mnxdom for later mnx schemas, so this PR should not be merged back to the main branch.

Hopefully this eliminates the need to exclude mnx from dev or production builds on any platform.

• I signed the CLA as username:
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• Each commit's message describes its purpose and effects, and references the issue it resolves. If changes are extensive, there is a sequence of easily reviewable commits.
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• There are no unnecessary changes.
• I created a unit test or vtest to verify the changes I made (if applicable).

[rpatters1]

If someone wants to tell me how, we could include removing the mnx exclusion in the build as part of this PR.

[juli27]

All that's left of the build exclusion (on both main and 4.7) are these lines:

MuseScore/SetupConfigure.cmake

Lines 373 to 379 in 0b3600b

	if (MUE_BUILD_IMPEXP_MNX_MODULE)
	find_program(XXD_PROGRAM xxd QUIET)
	if (NOT XXD_PROGRAM)
	message(WARNING "xxd not found, disabling MNX import/export module")
	set(MUE_BUILD_IMPEXP_MNX_MODULE OFF)
	endif()
	endif()

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[Jojo-Schmitz]

Could you prefix this PR's title with "[4.7.5]"?

[Jojo-Schmitz]

Tkanks.
Shouldn't the 2nd commit also be part of #33958 ?

[rpatters1]

Tkanks. Shouldn't the 2nd commit also be part of #33958 ?

I added it yesterday.