imapserver: implement NOTIFY extension from RFC 5465

NOTIFY is like IDLE, but where IDLE watches just the selected mailbox, NOTIFY
can watch all mailboxes. With NOTIFY, a client can also ask a server to
immediately return configurable fetch attributes for new messages, e.g. a
message preview, certain header fields, or simply the entire message.

Mild testing with evolution and fairemail.

webmail/api.json

@@ -2849,6 +2849,20 @@
 						"string"
 					]
 				},
+				{
+					"Name": "MessageCountIMAP",
+					"Docs": "For IMAP NOTIFY.",
+					"Typewords": [
+						"uint32"
+					]
+				},
+				{
+					"Name": "Unseen",
+					"Docs": "",
+					"Typewords": [
+						"uint32"
+					]
+				},
 				{
 					"Name": "MessageItems",
 					"Docs": "",
@@ -2968,6 +2982,27 @@
 						"[]",
 						"int64"
 					]
+				},
+				{
+					"Name": "UIDNext",
+					"Docs": "For IMAP NOTIFY.",
+					"Typewords": [
+						"UID"
+					]
+				},
+				{
+					"Name": "MessageCountIMAP",
+					"Docs": "",
+					"Typewords": [
+						"uint32"
+					]
+				},
+				{
+					"Name": "Unseen",
+					"Docs": "",
+					"Typewords": [
+						"uint32"
+					]
 				}
 			]
 		},
@@ -3017,6 +3052,20 @@
 						"[]",
 						"string"
 					]
+				},
+				{
+					"Name": "UIDValidity",
+					"Docs": "For IMAP NOTIFY.",
+					"Typewords": [
+						"uint32"
+					]
+				},
+				{
+					"Name": "Unseen",
+					"Docs": "",
+					"Typewords": [
+						"uint32"
+					]
 				}
 			]
 		},