Add setChartTitleFontColor() for independent title text color control

[timmolter]

Motivation

There was no way to set the chart title font color independently -- setChartFontColor() controls the title, axes labels, and legend all at once. Issue #706 asks for finer-grained control over just the title text color, consistent with the per-axis color overrides already available (e.g. setXAxisTitleColor()).

Approach

Added chartTitleFontColor to Styler as an optional override:

• getChartTitleFontColor() returns the override if set, otherwise falls back to chartFontColor -- fully backward compatible.
• setChartTitleFontColor(Color) is fluent and lives alongside the other chart title settings (setChartTitleFont, setChartTitleBoxBackgroundColor, etc.).
• ChartTitle now calls getChartTitleFontColor() instead of getChartFontColor() for both the regular text and TeX render paths.

Usage

chart.getStyler().setChartTitleFontColor(Color.RED);

// or combine with a custom font
chart.getStyler()
    .setChartTitleFontColor(new Color(220, 100, 0))
    .setChartTitleFont(new Font("Arial", Font.BOLD, 20));

Testing

• 4 unit tests in StylerChartTitleFontColorTest: default fallback, override, isolation from chartFontColor, and reset-to-null fallback.
• Demo: TestForIssue706 -- displays three charts side by side (default, red title, orange bold title) to visually verify the feature.