Fix a problem with multiple engines and engine removal

This fixes a problem that exists with engine removal in vehicles that have multiple engines installed.

In a vehicle with two engines installed, say A and B, you can do the following: set engine A to enabled, B to disabled, and then remove engine A. This leaves the vehicle with one disabled engine, and no way to re-enable it.

This patch adds a check for a situation where none of the remaining engines are enabled, and if that is the situation, it enables all remaining engines.