Expose a way to make visible or hidden a toolbar item

refs #4427 - In order to be able to hide the “Insert image” button when a WordPress user doesn’t have the upload_files capabilities, I need to be able to set the hidden property of the toolbar button item.
Because of the specifics of the implementation of WPEditorFormatbarView, rather than exposing a UIBarButtonItem* property to be consumed directly by the app, it’s better to provide a helper method that will, similarly to -enableToolbarItem:showSourceButton:, do the work under the hood of the editor itself.

Author: Baudouin

Classes/WPEditorFormatbarView.h

@@ -139,6 +139,14 @@ typedef enum
 
 #pragma mark - Toolbar items
 
+/**
+ *  @brief      Makes a toolbar item visible or hidden
+ *
+ *  @param      tag     WPEditorViewControllerElementTag of the item to alter.
+ *  @param      visible YES to make the item visible, NO to hide it.
+ */
+- (void)toolBarItemWithTag:(WPEditorViewControllerElementTag)tag setVisible:(BOOL)visible;
+
 /**
  *  @brief      Enables and disables the toolbar items.
  *

Classes/WPEditorFormatbarView.m

@@ -189,6 +189,21 @@ - (void)setSelectedItemTintColor:(UIColor *)selectedItemTintColor
 
 #pragma mark - Toolbar items
 
+- (void)toolBarItemWithTag:(WPEditorViewControllerElementTag)tag setVisible:(BOOL)visible
+{
+    for (ZSSBarButtonItem *item in self.leftToolbar.items) {
+        if (item.tag == tag) {
+            item.customView.hidden = !visible;
+        }
+    }
+
+    for (ZSSBarButtonItem *item in self.regularToolbar.items) {
+        if (item.tag == tag) {
+            item.customView.hidden = !visible;
+        }
+    }
+}
+
 - (void)enableToolbarItems:(BOOL)enable
     shouldShowSourceButton:(BOOL)showSource
 {