add resize handles between 3 main app columns

src/components/views/elements/ResizeHandle.js

@@ -0,0 +1,26 @@
+
+import React from 'react'; // eslint-disable-line no-unused-vars
+import PropTypes from 'prop-types';
+
+//see src/resizer for the actual resizing code, this is just the DOM for the resize handle
+const ResizeHandle = (props) => {
+    const classNames = ['mx_ResizeHandle'];
+    if (props.vertical) {
+        classNames.push('mx_ResizeHandle_vertical');
+    } else {
+        classNames.push('mx_ResizeHandle_horizontal');
+    }
+    if (props.reverse) {
+        classNames.push('mx_ResizeHandle_reverse');
+    }
+    return (
+        <div className={classNames.join(' ')}/>
+    );
+};
+
+ResizeHandle.propTypes = {
+    vertical: PropTypes.bool,
+    reverse: PropTypes.bool,
+};
+
+export default ResizeHandle;