surface: Add readouts of pitch, yaw and roll to F15's HUD.

This is of course not present in the real vehicle, but it's easier to
inspect if something wrong is happening.

Also note that we use lambda expression, feature introduced in C++11, for
the first time in this project.  This feature helps us keeping locality of
functions as high as possible.
surface: Directly calculate pitch and yaw from the quaternion.

This is not required, but would enhance performance (although may not
noticeable).  The theoretical solution should have the least arithmetic
errors and the least calculation cost, so we should utilize this
knowledge not only in F15's HUD.  This should be a good starting
point, though.
Add a flag to show or hide GLwindow system.

Clearing the flag in beginControl and restoring in endControl make the
GLwindow GUI elements hidden while the player is controlling an Entity.
This is useful to get rid of unusable GUIs that block player's view.
As always, F15 demonstrates this usage.
Add properties maxhealth for reading and health for writing in scripts.
surface: Make F15 capable of shooting missiles.

The Missile class is designed for space, so it doesn't behave
realistically in surface.  Specifically, it doesn't take effect of
gravity.
We should make a dedicated class, say AIM-9, for the real equipment.
Also we need a model for display.

For the time being, we reuse the logic of space missiles and it responds
well.

Also demo7.nut is made to restore health of the Entities with reset().
surface: Add armed weapon and missile count readings to F15's HUD.