-Codechange: Remove _avail_aircraft
Its name is misleading. It is rather _avail_airports, but then only some of them, which leads to inconsistencies when using it.
Further it is unnecessary to store it in savegams.
-The user will notice the following changes:
All vehicle types behaves in the same way
Adding/removing an engine (new design and so on) can no longer result in the window selecting a new engine
All valid replacements will be displayed since it looks at refitting options as well (this solves the missing ships with the newships GRF)
Since you can't replace an engine into itself, the engine selected in the left list will no longer appear in the right list
-The code changes:
Instead of looping all engines all the time, each list generates a list like the build windows
This ensures consistent list generation since only one function can loop all engines
Unified code for all vehicle types
It now use the lists to call the drawing code in the build vehicle window
Works on selected EngineIDs instead of selected index in the list
-Fix: Fix an out of bounds array access when assigning the engine type in the _engine array
This usually hits the _engine_name_strings array and causes an invalid StringID, though it depends on the compiler which data structure gets placed after the _engines array.
Most probably this was exposed by removing railtype from Engine, which changed the size of this struct.
