[Feature Request]: Support a single spanning title with a secondary subtitle in `g_forest()`

[wwojciech]

Feature description

Currently, g_forest() is capable of creating a double-text title for the plot.

However, for the tables and graphs I'm working on, the template requires:

• a single title/header spanning the entire forest plot,
• the double text displayed below the graph.

See the attached screenshot, below.

Would it be possible to extend g_forest() to support this as well?

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