Merge branch '46-improve-message-status-indicators' into 'master'

Improve how the status of messages is indicated.

Remove the Toast that always says 'Message Sent' and show graphical
indicators instead that show either:
* message is waiting to be sent
* message was sent (or requested to be sent)
* message was delivered

Please note that I didn't change the icons and did not migrate the UI to XML files to keep my change minimally invasive.

Closes #46

See merge request !9