Introduce a color for selected rows in data tables

- Rename unselectedColor to unselectedWidgetColor.

- Rename selectionColor to textSelectionColor.

- Add selectedRowColor.

- Remove hintOpacity since it's not tested and has no demo clients.

- Add some docs.

packages/flutter/lib/src/material/checkbox.dart

@@ -65,7 +65,7 @@ class Checkbox extends StatelessWidget {
     return new _CheckboxRenderObjectWidget(
       value: value,
       activeColor: activeColor ?? themeData.accentColor,
-      inactiveColor: onChanged != null ? themeData.unselectedColor : themeData.disabledColor,
+      inactiveColor: onChanged != null ? themeData.unselectedWidgetColor : themeData.disabledColor,
       onChanged: onChanged
     );
   }

packages/flutter/lib/src/material/input.dart

@@ -189,8 +189,8 @@ class _InputState extends State<Input> {
         focusKey: focusKey,
         style: textStyle,
         hideText: config.hideText,
-        cursorColor: themeData.selectionColor,
-        selectionColor: themeData.selectionColor,
+        cursorColor: themeData.textSelectionColor,
+        selectionColor: themeData.textSelectionColor,
         keyboardType: config.keyboardType,
         onChanged: onChanged,
         onSubmitted: onSubmitted

packages/flutter/lib/src/material/radio.dart

@@ -69,7 +69,7 @@ class Radio<T> extends StatelessWidget {
   bool get _enabled => onChanged != null;
 
   Color _getInactiveColor(ThemeData themeData) {
-    return _enabled ? themeData.unselectedColor : themeData.disabledColor;
+    return _enabled ? themeData.unselectedWidgetColor : themeData.disabledColor;
   }
 
   void _handleChanged(bool selected) {

packages/flutter/lib/src/material/two_level_list.dart

@@ -147,7 +147,7 @@ class _TwoLevelSublistState extends State<TwoLevelSublist> {
       ..begin = theme.textTheme.subhead.color
       ..end = theme.accentColor;
     _iconColor
-      ..begin = theme.unselectedColor
+      ..begin = theme.unselectedWidgetColor
       ..end = theme.accentColor;
 
     return new AnimatedBuilder(