Stubbed custom attribute adding via context menu

Made MPContextToolbarButton usable as normal button.
Added private API helper for adding buttons to NSUserNotification
Remove Autotype plugin API
Enhanced custom attribute plugin API
Enhanced EntryActionPlugin API to decouple menu items from actual action/target setup
Stubbed action on no-open documents NSUserNotifications
Stubbed HMACOTP custom attribute setup

MacPass/MPAppDelegate.m

@@ -38,6 +38,7 @@
 #import "MPPrettyPasswordTransformer.h"
 #import "MPTemporaryFileStorageCenter.h"
 #import "MPValueTransformerHelper.h"
+#import "MPUserNotificationCenterDelegate.h"
 
 #import "NSApplication+MPAdditions.h"
 
@@ -50,6 +51,7 @@ NSString *const MPDidChangeStoredKeyFilesSettings = @"com.hicknhack.macpass.MPDi
 @interface MPAppDelegate () {
 @private
   MPDockTileHelper *_dockTileHelper;
+  MPUserNotificationCenterDelegate *_userNotificationCenterDelegate;
   BOOL _shouldOpenFile; // YES if app was started to open a
 }
 
@@ -71,6 +73,7 @@ NSString *const MPDidChangeStoredKeyFilesSettings = @"com.hicknhack.macpass.MPDi
 - (instancetype)init {
   self = [super init];
   if(self) {
+    _userNotificationCenterDelegate = [[MPUserNotificationCenterDelegate alloc] init];
     /* We know that we do not use the variable after instantiation */
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunused-variable"