Add trusted state after OMEMO decryption

Use it to print message on red background if not trusted.

src/omemo/store.h

@@ -49,6 +49,7 @@ typedef struct {
    uint32_t registration_id;
    GHashTable *trusted;
    bool recv;
+   bool trusted;
 } identity_key_store_t;
 
 GHashTable * session_store_new(void);