Remove the vessel release context menu item.

I had forgotten to remove this when I rewrote the gui code for 3.7.
Do not save the base mass if it is 0.

This happens when saving a craft file in the VAB or SPH and causes the
part's mass to be 0 in-game (a very bad thing).
Set the state to Transfer only in timed-builds mode.

This fixes the inability to release built vessels in insta-build mode
(Transfer is not used in insta-build).

Fixes #48.