Peek reipients, fetch existing messages

Read all of an e-mail accounts messages and extract all To/CC addresses
if the From was from our own account.
Then, fetch existing messages from the server and show them.

Also, I fixed two other things:
- just by chance my test failed because of an completely unrelated bug.
The bug: bcc_self messages were not marked as read if mvbox_move was set
to true.
- add some color to the test output (minor change)

src/constants.rs

@@ -195,6 +195,9 @@ pub const DC_LP_AUTH_NORMAL: i32 = 0x4;
 /// if none of these flags are set, the default is chosen
 pub const DC_LP_AUTH_FLAGS: i32 = DC_LP_AUTH_OAUTH2 | DC_LP_AUTH_NORMAL;
 
+/// How many existing messages shall be fetched after configuration.
+pub const DC_FETCH_EXISTING_MSGS_COUNT: i64 = 100;
+
 // max. width/height of an avatar
 pub const AVATAR_SIZE: u32 = 192;