Rename popup.h to PopupMenu.h for better indication of defined class

I know there are a lot of misnomers left, but it's a good starting point.
Add PopupMenu-forward.h, a forward declaration header

Also added reference to glw header files to VC2012's project file.
Add mechanism to load popup menu items from Squirrel scripts to Entity

Each time the player clicks right mouse button on the screen,
Entity::popupMenu() tries to load a list of popup menu items from a
executed result of a Squirrel function.  This way, we can fully customize
the contents of the menu without recompiling.  In addition, we have
opportunity to dynamically tweak the menu items.

As a demonstration, Sceptor utilizes this mechanism.

The overhead of executing scripted function is negligible, because it's so
rare that the method is invoked.
Make CoordSys aware of compare operator in Squirrel scripts

The comparison operator metamethods do not affect equality operators
(==,!=), so this change won't make equality operators return true when
applied to the same CoordSys object returned from multiple sources
(even the expresson "player.cs==player.cs" return false).  This problem
should be addressed in some time.

Also added "id" read-only property to CoordSys class in Squirrel script.
Make Player::fov parameter accessible from Squirrel scripts
Brush up Player's Squirrel parameter accessors

* Convert strcmp() to scstrcmp() for interpreting parameter string for
  Unicode portability.
* Make sq_throwerror strings be more descriptive about the error type.
* Fix some return statements in sq_get() to return 0 instead of 1.
* Fix some comments' grammer.
Add mechanism to run cockpitView Squirrel script handler

This mechanism follows popupMenu's idea.  The Squirrel callback function
returns a table containing members named "pos" and "rot", which indicate
desired position and rotation of the camera, respectively.

Just like popupMenu, Sceptor demonstrates this change.