Performance improvements

I've made some performance improvements.

Statistics of times the message is parsed with the email library per mail object:

| View                                    |        Before |  After |
| -------------                           | ------------- | ------ |
| Details view                            |            14 |      1 |
| Tabbed view mails listing (1 access)    |             4 |      3 |
| Tabbed view mails listing (2..n access) |             4 |      0 |

I've added persistent (!) caches, which are set / invalidated when setting the message:
- Title
- Attachment informations
- Headers (on access)