Add an "Edit Mode" to Dashboard view (#3940)

* Add a togglable edit mode to dashboard

* Submenu for controls

* Allowing 'Save as' outside of editMode

* Set editMode to false as default

superset/assets/javascripts/dashboard/components/GridLayout.jsx

@@ -28,6 +28,7 @@ const propTypes = {
   getFilters: PropTypes.func,
   clearFilter: PropTypes.func,
   removeFilter: PropTypes.func,
+  editMode: PropTypes.bool.isRequired,
 };
 
 const defaultProps = {
@@ -162,6 +163,7 @@ class GridLayout extends React.Component {
             getFilters={this.props.getFilters}
             clearFilter={this.props.clearFilter}
             removeFilter={this.props.removeFilter}
+            editMode={this.props.editMode}
           />
         </div>);
     });