Support setting button font family on Android

Until now, buttons would use the same font family used by the title,
this commit introduces navBarButtonFontFamily property which allows to set
a different font family for title and buttons.

android/app/src/main/java/com/reactnativenavigation/params/StyleParams.java

     public boolean titleBarTitleTextCentered;
     public int titleBarHeight;
     public boolean backButtonHidden;
+    public Font titleBarButtonFontFamily;
 
     public Color topTabTextColor;
     public Color topTabIconColor;

android/app/src/main/java/com/reactnativenavigation/params/TitleBarButtonParams.java

     @Override
     public void setStyleFromScreen(StyleParams styleParams) {
         super.setStyleFromScreen(styleParams);
-        font = styleParams.titleBarTitleFont;
+        font = styleParams.titleBarButtonFontFamily.hasFont() ? styleParams.titleBarButtonFontFamily : styleParams.titleBarTitleFont;
     }
 
     public boolean hasFont() {

android/app/src/main/java/com/reactnativenavigation/params/parsers/StyleParamsParser.java

         result.titleBarSubtitleFontSize = getInt("titleBarSubtitleFontSize", getDefaultSubtitleTextFontSize());
         result.titleBarSubtitleFontFamily = getFont("titleBarSubtitleFontFamily", getDefaultSubtitleFontFamily());
         result.titleBarButtonColor = getColor("titleBarButtonColor", getTitleBarButtonColor());
+        result.titleBarButtonFontFamily = getFont("titleBarButtonFontFamily", getDefaultTitleBarButtonFont());
         result.titleBarDisabledButtonColor = getColor("titleBarDisabledButtonColor", getTitleBarDisabledButtonColor());
         result.titleBarTitleFont = getFont("titleBarTitleFontFamily", getDefaultTitleTextFontFamily());
         result.titleBarTitleFontSize = getInt("titleBarTitleFontSize", getDefaultTitleTextFontSize());
         result.bottomTabFontFamily = new StyleParams.Font();
         result.titleBarTitleFont = new StyleParams.Font();
         result.titleBarSubtitleFontFamily = new StyleParams.Font();
+        result.titleBarButtonFontFamily = new StyleParams.Font();
         result.titleBarHeight = -1;
         return result;
     }
         return AppStyle.appStyle == null ? new StyleParams.Font() : AppStyle.appStyle.titleBarSubtitleFontFamily;
     }
 
+    private StyleParams.Font getDefaultTitleBarButtonFont() {
+        return AppStyle.appStyle == null ? new StyleParams.Font() : AppStyle.appStyle.titleBarButtonFontFamily;
+    }
+
     private boolean getDefaultTitleTextFontBold() {
         return AppStyle.appStyle != null && AppStyle.appStyle.titleBarTitleFontBold;
     }

docs/styling-the-navigator.md

   // Android only
   navigationBarColor: '#000000', // change the background color of the bottom native navigation bar.
   navBarTitleTextCentered: true, // default: false. centers the title.
+  navBarButtonFontFamily: 'sans-serif-thin', // Change the font family of textual buttons
   topBarElevationShadowEnabled: false, // default: true. Disables TopBar elevation shadow on Lolipop and above
   statusBarColor: '#000000', // change the color of the status bar.
   collapsingToolBarImage: "http://lorempixel.com/400/200/", // Collapsing Toolbar image.

src/deprecated/platformSpecificDeprecated.android.js

     titleBarSubtitleFontSize: originalStyleObject.navBarSubtitleFontSize,
     titleBarSubtitleFontFamily: originalStyleObject.navBarSubtitleFontFamily,
     titleBarButtonColor: processColor(originalStyleObject.navBarButtonColor),
+    titleBarButtonFontFamily: originalStyleObject.navBarButtonFontFamily,
     titleBarDisabledButtonColor: processColor(originalStyleObject.titleBarDisabledButtonColor),
     titleBarTitleFontFamily: originalStyleObject.navBarTextFontFamily,
     titleBarTitleFontSize: originalStyleObject.navBarTextFontSize,