plot gridline visibility, got rid of IHideable

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

...in/java/com/xeiam/xchart/internal/chartpart/AxisTick.java

...n/java/com/xeiam/xchart/internal/chartpart/AxisTitle.java

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

...main/java/com/xeiam/xchart/internal/chartpart/Legend.java

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

.../java/com/xeiam/xchart/internal/interfaces/IHideable.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