Is drag the only aerodynamic force that acts upon an asymmetric body (a glider in particular) travelling through the air?
You are right, lift and drag are just convention. But the aerodynamic forces are usually grouped into one total force named "total aerodynamic force" ("résultante aérodynamique" in french).

As explain in how it flies:


  
  drag is a component of the aerodynamic force, namely the projection onto the direction parallel to the relative wind.
  lift is another component of the aerodynamic force, namely the projection onto the two directions perpendicular to the relative wind. 
  


This convention applies to any aircraft.

For further reading, see the section about forces of how it flies.