Changed external intent (e.g. Intent.ACTION_SEND_TO) handling code in MessageCompose

- extracted handling of external intents to a new method
- only allow mailto URI for ACTION_VIEW and ACTION_SENDTO (as documented by AOSP)
- allow additional recipients via "to" parameter in mailto URI
- removed check for allowed MIME type when adding attachments since we're accepting all MIME types anyway
- show CC and BCC text fields when one of those recipient types was set by the intent
- use MIME type specified in intent for attachments (if present and doesn't contain a "*")