Feat: Different types of legend icon

[brenocq]

Closes #569
Closes #160

Improved Legend Icons

Summary

Legend icons now accurately represent the plot type instead of always showing a solid square.

Changes

• Line plots: Display as horizontal lines in legend
• Line plots with markers: Display as horizontal line with centered marker
• Scatter plots: Display as single marker (no line)
• Filled plots (shaded, pie, heatmap, digital): Display as filled square
• Bar plots/histograms: Display as filled square with outline

Implementation

• Added ImPlotLegendIconType_ enum with 5 icon types: Line, LineMarkers, Markers, Fill, FillLine
• Extended ImPlotItem to store legend rendering data (colors, marker type, line weight, icon type)
• Added RenderLegendIcon() to render icons based on type
• Each plot function explicitly sets its icon type when calling BeginItem()
• Markers in legend are consistently sized at 0.5× icon width

Technical Details

• Icon type is determined per-plot-type
• Legend icons respect the same color properties as the plot (line color, fill color, marker colors)
• Icons respond to hover/held/disabled states with alpha modulation
• No breaking changes to public API

[janarombe]

Isn't there a problem in the "Stairstep Plot" picture above? The "Post Step (default)" legend item shows a blue square, but in the curve, the markers look like circles.