fixes #1322

Slider used to skip back on drag-and-drop action. The switch doesn't
trigger explicit whole UI refreshes anymore, it now directly adjusts the
"done" button's locked status.

XmppAxolotlMessage is now entirely responsible for handling encryption
and decryption of messages, only leveraging XmppAxolotlSession as a
packing/unpacking primitive for payload keys.

Removed pseudo-dead session generation code step from prepareMessage
function, as sessions have been created by invoking the
TrustKeysActivity for a while now.

Added prepareKeyTransportMessage function, which creates a message with
no payload. The key that is packed into the header keyElements can then
be used for other purposes (e.g. encrypted file transfer).

fixes #1310

removed expert setting to 'force encryption'. merged certifcate and encryption settings into same category

Messages sent from another device of the own account are now explicitly
tagged as carboned message. The session detection logic now uses this
tag to find "session borders".

Plaintext messages that were received while in an encrypted session are
now colored red. We define "in an encrypted session" if a) the last
message sent by our own device before the message under consideration
(or any message received between then and now) was encrypted AND b) the
next message will be sent encrypted or the next message sent after the
one under consideration was sent encrypted