App extension - Improvement: Reduce memory consumption.

Force the ShareExtensionManager to release the primary view controller when the action is completed.

We observed that `dealloc` is called for this view controller when the request is cancelled with error (`cancelRequestWithError`), but it is not called when the request is completed (`completeRequestReturningItems`).

RiotShareExtension/Model/ShareExtensionManager.h

@@ -18,6 +18,7 @@
 #import <MatrixKit/MatrixKit.h>
 
 @class ShareExtensionManager;
+@class SharePresentingViewController;
 
 /**
  Posted when the matrix session has been changed.
@@ -64,6 +65,11 @@ extern NSString *const kShareExtensionManagerDidChangeMXSessionNotification;
  */
 @property (nonatomic) NSExtensionContext *shareExtensionContext;
 
+/**
+ The share app extension’s primary view controller.
+ */
+@property (nonatomic) SharePresentingViewController *primaryViewController;
+
 /**
  The associated matrix session (nil by default).
  */