HyperlinkBase Property, and Html Handling of Properties

[oleibman]

Fix #3573. The original issue concerned non-support of Document Properties in Xml spreadsheets. However, most of the Properties mentioned there were already supported. But the investigation revealed some gaps in Html coverage.

HyperlinkBase is the one property mentioned in the issue that was not supported for Xml, nor indeed for any other format. All the other document properties are 'meta'; but HyperlinkBase is functional - if you supply a relative address for a link, Excel will use HyperlinkBase, if supplied, to convert to an absolute address. (Default is directory where spreadsheet is located.) Here's a summary of how this PR will handle this property for various formats:

• Support is added for Xlsx read and write.
• Support is added for Xml read (there is no Xml writer). Ironically, Excel messes up this processing when reading an Xml spreadsheet; however, PhpSpreadsheet will get it right.
• HyperlinkBase is supported for Xls, but I have no idea how to read or write this property. For now, when writing hyperlinked cells, PhpSpreadsheet will be changed to convert any relative addresses that it can detect to absolute references by adding HyperlinkBase to the relative address. In a similar vein, Xls supports custom properties, but PhpSpreadsheet does not know how to read or write those.
• Gnumeric has no equivalent property, so nothing needs to be done to its reader. Since we don't have a Gnumeric writer, that's not really a problem for us.
• Odt has no equivalent property, so nothing needs to be done to its reader. The Odt writer does not have any special logic for hyperlinks, so, at least for now, will remain unchanged.
• Csv has no equivalent property, so nothing needs to be done to its reader. The Csv writer does not have any special logic for hyperlinks, so, at least for now, will remain unchanged.
• Html allows for an equivalent base tag in the head section. Support for this is added to Html reader and writer.

Html Writer was only handling 8 of the 11 'core' properties. Support is added for created, modified, and lastModifiedBy. Custom properties were not supported at all, and now are.

Html Reader did not support any properties. It will now support all of them.

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.

[oleibman]

Fixed minor Scrutinizer problem. No intent to fix "complexity" problem.