Add support for macOS 10.13

The problem is that macOS 10.13 does not allow to simply mutate `items`
in menu. You have to use addItem/removeItem instead. The support for
direct mutation was added in Mojave.

https://developer.apple.com/documentation/macos_release_notes/macos_mojave_10_14_release_notes/appkit_release_notes_for_macos_10_14

Maccy.xcodeproj/project.pbxproj

@@ -612,7 +612,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				MACOSX_DEPLOYMENT_TARGET = 10.14;
+				MACOSX_DEPLOYMENT_TARGET = 10.13;
 				MTL_ENABLE_DEBUG_INFO = YES;
 				ONLY_ACTIVE_ARCH = YES;
 				SDKROOT = macosx;
@@ -664,7 +664,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				MACOSX_DEPLOYMENT_TARGET = 10.14;
+				MACOSX_DEPLOYMENT_TARGET = 10.13;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				SDKROOT = macosx;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";

Maccy/Menu.swift

@@ -20,7 +20,12 @@ class Menu: NSMenu, NSMenuDelegate {
   }
 
   func menuWillOpen(_ menu: NSMenu) {
-    self.items = allItems
+    removeAllItems()
+
+    let oldAllItems = allItems
+    oldAllItems.forEach(addItem(_:))
+    allItems = oldAllItems
+
     setKeyEquivalents(items)
     highlight(highlightableItems(items).first)
   }
@@ -48,16 +53,18 @@ class Menu: NSMenu, NSMenuDelegate {
   }
 
   func updateFilter(filter: String) {
+    removeAllItems()
+
     let oldAllItems = allItems
 
     let searchItem = allItems.first
     let results = search.search(string: filter, within: searchableItems())
     let systemItems = allItems.filter({ isSystemItem(item: $0) })
 
-    items = [searchItem!]
-    items += results
-    items += [NSMenuItem.separator()]
-    items += systemItems
+    addItem(searchItem!)
+    results.forEach(addItem(_:))
+    addItem(NSMenuItem.separator())
+    systemItems.forEach(addItem(_:))
 
     allItems = oldAllItems