Fix #658: rename tick-label formatter styler methods to proper Java case

[timmolter]

Problem

Issue #658 asks whether a custom tick-label API exists. It does — but users couldn't find it because setxAxisTickLabelsFormattingFunction starts with a lowercase x, making it invisible in IDE autocompletion next to all the other setX… / setY… styler methods.

To make things worse, the nicer convenience methods setCustomXAxisTickLabelsFormatter / setCustomYAxisTickLabelsFormatter on AxesChart were marked @Deprecated, pushing users toward the hard-to-find styler methods.

Changes

AxesChartStyler

• Add setXAxisTickLabelsFormattingFunction / getXAxisTickLabelsFormattingFunction (uppercase X — proper Java convention)
• Add setYAxisTickLabelsFormattingFunction / getYAxisTickLabelsFormattingFunction (uppercase Y)
• Deprecate the old lowercase setxAxis… / setyAxis… variants; they now delegate to the new methods (no behaviour change)

AxesChart

• Remove the erroneous @Deprecated tag from setCustomXAxisTickLabelsFormatter / setCustomYAxisTickLabelsFormatter
• Add proper Javadoc to both
• Update internal calls to the new styler method names

Internal callers

• Axis_X, Axis_Y updated to call the new getter names

Demos & tests

• All existing callsites updated to the new names
• TestForIssue658.java added — shows the map-to-label pattern (build a Map<Double, String> and pass a lookup lambda as the formatter)

Usage after this fix

// Via the chart convenience method (recommended)
chart.setCustomXAxisTickLabelsFormatter(x -> myLabels.getOrDefault(x, ""));

// Or directly via the styler
chart.getStyler().setXAxisTickLabelsFormattingFunction(x -> myLabels.getOrDefault(x, ""));

Closes #658