Do not skip Sent and Spam folders on Gmail

Skipping of all [Gmail] folders was introduced to avoid scanning
virtual "[Gmail]/All Mail" folder. However, it also skips Sent and
Spam folders which reside inside [Gmail]. As a result
configured_sentbox_folder becomes unset after folder scan, making it
impossible to watch Sent folder, and Spam folder is never scanned.

This change makes Delta Chat identify virtual Gmail folders by their
flags, so virtual folders are skipped while Sent and Spam folders are
still scanned.

src/imap.rs

@@ -125,6 +125,14 @@ enum FolderMeaning {
     Sent,
     Drafts,
     Other,
+
+    /// Virtual folders.
+    ///
+    /// On Gmail there are virtual folders marked as \\All, \\Important and \\Flagged.
+    /// Delta Chat ignores these folders because the same messages can be fetched
+    /// from the real folder and the result of moving and deleting messages via
+    /// virtual folder is unclear.
+    Virtual,
 }
 
 impl FolderMeaning {
@@ -135,6 +143,7 @@ impl FolderMeaning {
             FolderMeaning::Sent => Some(Config::ConfiguredSentboxFolder),
             FolderMeaning::Drafts => None,
             FolderMeaning::Other => None,
+            FolderMeaning::Virtual => None,
         }
     }
 }
@@ -1888,6 +1897,7 @@ fn get_folder_meaning(folder_name: &Name) -> FolderMeaning {
                 "\\Sent" => return FolderMeaning::Sent,
                 "\\Spam" | "\\Junk" => return FolderMeaning::Spam,
                 "\\Drafts" => return FolderMeaning::Drafts,
+                "\\All" | "\\Important" | "\\Flagged" => return FolderMeaning::Virtual,
                 _ => {}
             };
         }