Add keybound GrowHUD/ShrinkHUD (+/- keys by default) functionality to vehicles

[Matt Hands]

HUD class has keybound exec functions to grow or shrink the HUD display. These keys/functions do nothing when the player is in a vehicle. Spare keybound functions are quite rare and are precious, so this is a waste. This pair are especially useful, as the keys are +/- by default.

I have modified the HUD execs to call same named functions in the relevant vehicle classes if the player is in a vehicle. Then I have added empty functions to those vehicle classes.

GrowHUD and ShrinkHUD functions can now be modified in vehicle subclasses to any an desired custom functionality. This will be especially useful for smoke launchers, to adjust range or aim.

[Matt Hands]

Implemented in commit 3dc5056

[Matt Hands]

Hmmmm ...... modified GrowHUD & ShrinkHUD functions do not compile due to circular dependencies.

Believe problem stems from DH abstract vehicle classes being in DH_Vehicles, where RO abstract vehicle classes are in ROEngine. For example, ROHud references ROTreadCraft without a problem.

This is quite limiting and will affect several features. Propose to move DH abstract vehicle classes to DH_Engine and find & update all references to full package.class names.

[Matt Hands]

Re-implemented in commit d339768, which moves lots of abstract & associated vehicle classes from DH_Vehicles to DH_Engine