Added keywords to header condition

FAQ.md

@@ -2363,6 +2363,23 @@ Some common header conditions (regex):
 * *.*Auto-Submitted:.** [RFC3834](https://tools.ietf.org/html/rfc3834)
 * *.*Content-Type: multipart/report.** [RFC3462](https://tools.ietf.org/html/rfc3462)
 
+You can match IMAP flags (keywords) via a header condition too (from version 1.1777), like this:
+
+```
+$<keyword>$
+```
+
+You can use these special values too, representing common system flags:
+
+```
+$$seen$
+$$answered$
+$$flagged$
+$$deleted$
+```
+
+Note that *regex* should be disable and that there should be no white space.
+
 In the three-dots *more* message menu there is an item to create a rule for a received message with the most common conditions filled in.
 
 The POP3 protocol does not support setting keywords and moving or copying messages.

app/src/main/java/eu/faircode/email/EntityRule.java

@@ -239,6 +239,26 @@ public class EntityRule {
                 String value = jheader.getString("value");
                 boolean regex = jheader.getBoolean("regex");
 
+                if (!regex &&
+                        value != null &&
+                        value.startsWith("$") &&
+                        value.endsWith("$")) {
+                    String keyword = value.substring(1, value.length() - 1);
+
+                    List<String> keywords = new ArrayList<>();
+                    if (message.ui_seen)
+                        keywords.add("$seen");
+                    if (message.ui_answered)
+                        keywords.add("$answered");
+                    if (message.ui_flagged)
+                        keywords.add("$flagged");
+                    if (message.ui_deleted)
+                        keywords.add("$deleted");
+                    keywords.addAll(Arrays.asList(message.keywords));
+
+                    if (!keywords.contains(keyword))
+                        return false;
+                } else {
                     boolean matches = false;
                     Enumeration<Header> headers;
                     if (imessage != null)
@@ -259,6 +279,7 @@ public class EntityRule {
                     if (!matches)
                         return false;
                 }
+            }
 
             // Date
             JSONObject jdate = jcondition.optJSONObject("date");