Recognize 'Hidden' Attribute and Other Unsupported Options in Xml Spreadsheet

[oleibman]

Fix #3566. Fix #3568. Fix #3569. PhpSpreadsheet should now handle Hidden attribute for rows and columns in Xml spreadsheets. Also added the ability to load Xml spreadsheet from string rather than file, as can be done for Csv and Html.
Also:

• top left Cell
• check value for font Bold/Italic rather than always setting to true
• read active sheet index from Xml
• set selected cell(s) on each worksheet
• worksheet protection (sheet only)
• style protection
• better handling of default style
• column spans in xml

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.