gundam: Insert intermediate MobileSuit class to share codes

ZetaGundam and ReZEL share many common line of codes, so we should
collect them in one place.  The MobileSuit class seems conceptually
straightforward.
gundam: Enable weapon switch with numeric keys for Mobile Suits

Also we added a .nut file for common codes among ReZEL and ZetaGundam.
gundam: Add MobileSuit-draw.cpp to share drawHUD() method

In this process, we added some virtual getter functions.
gundam: Add weapon list to HUD for MobileSuit.
gundam: Make MobileSuit derived classes have own list of weapons

The subclasses of MobileSuit should be able to have distinct weapons,
but they couldn't before this commit.

Now the subclasses can return weapon parameters with getWeaponParams()
to have their own list of weapons.
gundam: Draw boost and armor gauge to the HUD of MobileSuits

The subclasses of MobileSuit do not draw health bar on top of the
objects as overlays if that information is shown in HUD, because the
bars tend to get in the way of view.
gundam: Make ZetaGundam's overlay icon specific to the Entity type