Add cancel button for EIP connection.

Closes: #4035

This falls in the "quick" workaround category.

A proper state machine that extends the four basic connection states
is hence needed. We have to accomodate design to have a
connection-oriented state machine in the backend (I would favor a
twisted protocol for this), and a more lightweight one that conducts
the gui-level changes (ie, change the actions / buttons / labels
accordingly).

Since this "cancel" functionality has been long postponed, I chose
to do one more ugly hack here, that is, show and hide dance with a
button that just calls the bitmask-root to kill the vpn process. It
should work well enough until we get to the reorganization needed for a
clean process control for eip.