Translations: Replace line numbers with occurrence count in source TS

[shoogle]

Makes PRs like this one easier to review:

• Update in-repo translation source files #34553

That PR added 9 strings but they are buried in the diff, which mostly shows unimportant changes to line numbers.

By removing line numbers from TS files (or replacing them with an occurrence count, which rarely changes), future translation PRs will have a smaller diff that only includes relevant information.

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Here's what the current PR actually does to the *_en.ts files.

Extract from share/locale/instruments_en.ts

Filenames are now relative to the repository root. Each string occurs once so the line number is now always 1.

Extract from share/locale/musescore_en.ts

Filenames are now relative to the repository root. "Undo" & "Redo" each appear 4 times in notationuiactions.cpp so their line number is now 4 for that file, and 1 for other files where the strings only appear once.

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It's only necessary to update the *_en.ts files. After these are uploaded to Transifex, the changes will propagate to TS files in other languages.

None of these changes impact Qt tools. While lupdate does read our modified TS file, it only does so to:

1. Determine the order in which strings should appear in the updated TS file.
2. Preserve any obsolete strings that were in the old TS file but are no longer in the C++ source code.

Crucially, lupdate replaces all location data in the old TS file with fresh location data extracted from the C++ source code (or no location in the case of obsolete strings), so our modified locations are never used.

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Alternative solution:

• Translations: Round source TS line numbers up to nearest 10 #34613