chart legend style refactor

.../src/main/java/com/xeiam/xchart/demo/charts/Example1.java

...src/main/java/com/xeiam/xchart/demo/charts/Example10.java

.../src/main/java/com/xeiam/xchart/demo/charts/Example2.java

.../src/main/java/com/xeiam/xchart/demo/charts/Example8.java

.../src/main/java/com/xeiam/xchart/demo/charts/Example9.java

xchart/src/main/java/com/xeiam/xchart/Chart.java

xchart/src/main/java/com/xeiam/xchart/QuickChart.java

...c/main/java/com/xeiam/xchart/internal/chartpart/Axis.java

...java/com/xeiam/xchart/internal/chartpart/ChartLegend.java

...rt/src/main/java/com/xeiam/xchart/style/StyleManager.java

xchart/src/main/java/com/xeiam/xchart/style/theme/Theme.java

...c/main/java/com/xeiam/xchart/style/theme/XChartTheme.java