feat(DropdownMenu): allow menu to persist

Add persist prop to DropdownMenu. When true, the DropdownMenu will always render a Popper and not remove it when hidden.
This prop allows the children to persist their state and not be re-initialized when the menu opens/closes.

Fixes #779

docs/lib/Components/DropdownsPage.js

@@ -90,7 +90,8 @@ DropdownMenu.propTypes = {
   className: PropTypes.string,
   cssModule: PropTypes.object,
   // Custom modifiers that are passed to DropdownMenu.js, see https://popper.js.org/popper-documentation.html#modifiers
-  modifiers: PropTypes.object
+  modifiers: PropTypes.object,
+  persist: PropTypes.bool // presist the popper, even when closed. See #779 for reasoning
 };
 
 DropdownItem.propTypes = {

src/DropdownMenu.js

@@ -12,6 +12,7 @@ const propTypes = {
   modifiers: PropTypes.object,
   className: PropTypes.string,
   cssModule: PropTypes.object,
+  persist: PropTypes.bool,
 };
 
 const defaultProps = {
@@ -47,7 +48,7 @@ const DropdownMenu = (props, context) => {
 
   let Tag = tag;
 
-  if (context.isOpen && !context.inNavbar) {
+  if (persist || (context.isOpen && !context.inNavbar) {
     Tag = Popper;
 
     const position1 = directionPositionMap[context.direction] || 'bottom';