fix #3666, fix #3588, fix #3738

[sramazzina]

fix #3666 - DuckDB - Explorer shows multiple entries for schemas. Tables and Views not populated in their sections.
fix #3588 - Show layout in database explorer don't work
fix #3738 - DuckDB ALTER COLUMN statement not working

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[sramazzina]

The show layout dialog must be completely rewritten because the current dialog is completely out of context. In any case the issue is now fixed

[sramazzina]

I put the PR in draft waiting to do more tests with some commonly used databases.

[sramazzina]

Tested on MySQL, PostgreSQL, DuckDB and SQL Server with success. In my opinion this PR can be merged