Navigation: Add possibility to instantiate each tab bar item controller programmatically.

Riot/Modules/GlobalSearch/UnifiedSearchViewController.h

@@ -25,6 +25,8 @@
  */
 @interface UnifiedSearchViewController : SegmentedViewController <UIGestureRecognizerDelegate, ContactsTableViewControllerDelegate>
 
++ (instancetype)instantiate;
+
 /**
  Open the public rooms directory page.
  It uses the `publicRoomsDirectoryDataSource` managed by the recents view controller data source

Riot/Modules/GlobalSearch/UnifiedSearchViewController.m

@@ -57,6 +57,13 @@
 
 @implementation UnifiedSearchViewController
 
++ (instancetype)instantiate
+{
+    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"Main" bundle:[NSBundle mainBundle]];
+    UnifiedSearchViewController *viewController = [storyboard instantiateViewControllerWithIdentifier:@"UnifiedSearchViewController"];
+    return viewController;
+}
+
 - (void)finalizeInit
 {
     [super finalizeInit];