Font Themes

[oleibman]

It isn't a feature of Excel that I've made any use of, but PR #3476 added better Theme support for colors, and it is relatively easy to add Theme support for Fonts on top of that.

Excel assigns two theme fonts to its spreadsheets, one for Headings (major), and one for Body (minor). If the body theme is Calibri, when you choose a font for a cell in Excel, you can choose 'Calibri (Body)' from the Theme Fonts section at the top of the Font dropdown, or 'Calibri' from the 'All Fonts' section. If you choose the former, the cell will be automatically restyled if you change the Theme Fonts (via Page Layout, Themes, Fonts). The relationship to the theme fonts is recorded in the XML via a scheme tag (descending from font) whose val attribute can be either major or minor. Accordingly, this PR, in addition to defining the Theme Font properties, adds a scheme property, with getter and setter, to Style/Font.

The main benefit of this PR is that you can now load and save a spreadsheet preserving the connections to the Theme Fonts, without having to take any additional action.

A secondary benefit arises from the following difference. Empty cells in Excel will use the spreadsheet's default font name when they are filled in; but, in Google Sheets, they will use the Theme Minor Font name. By setting the scheme property in the default style, the resulting spreadsheet will behave the same in both Excel and Google.

I will note that Excel's font themes specify a Latin font, an East Asian font, a Complex Scripts font, and a set of font substitutions for various languages. PhpSpreadsheet will preserve all of these, and allow them to be changed. However, although it is easy to imagine how the non-Latin options might work, I have not yet been able to come up with an example where Excel uses any of them. In particular, if I use a theme font which does not support language X, and I use Language X in a cell bound to the theme, Excel will use a substitution font which does support it, but the font which it uses does not seem to be chosen from the alternatives supplied in the theme.

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).
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