Transport selection

Replace ContextMenu with popup to temporarily change preferred transport on longpress of send button.

![screenshot](https://cloud.githubusercontent.com/assets/373823/3654788/8b2549d0-1170-11e4-8ea2-70ec26f0abe9.png)

Without this, any onResume will set the transport to the default option, so we need to know when the user has selected something manually and we shouldn't reset it.