add legend title

[bircni]

This pull request introduces enhancements to the Legend functionality in the egui_plot library, updates the plot demos to include legend titles, and modifies related tests and snapshots to reflect these changes. The most significant changes include adding a title property to the Legend struct, updating plot demos to utilize this new feature, and adjusting test infrastructure and snapshots accordingly.

Enhancements to Legend functionality:

• egui_plot/src/legend.rs: Added a title property to the Legend struct, with a default value of None. Introduced a new title method to set the legend title and updated the LegendWidget to display the title if it is set. [1] [2] [3] [4]

Updates to plot demos:

• demo/src/plot_demo.rs: Updated the Legend configuration in LineDemo, MarkerDemo, and ItemsDemo to include titles ("Lines", "Markers", and "Items", respectively). [1] [2] [3]

• Closes Title (or label) for legend. #98

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