qmake|OS X 10.8: Ignore warnings in Qt headers

Unused private fields.
libdeng2|Block: Constructor from a null terminated C-string
Fixed|libdeng2: Improvements to the Observers template

It is not a good idea to release the _members set at any time; it
might be in use in loop iteration when the last member is removed.
Now _members is deleted only in the Loop destructor.
Fixed|libdeng2: AssetGroup must observe state changes in members

The state of the group depends on the state of the members.
libgui|Drawable: Avoid unnecessary notifications when deleting contents

Remove the asset first from the group, then delete.
Fixed|libgui: Handling failed GLShader construction

GLShader is reference counted, which means it cannot be destroyed
unless its reference count is zero. A failed constructor must reset
the default reference held in the constructor of Counted.
libgui|GLProgram|GLUniform: Usability improvements

Easier method for building a program out of two vtx+frag sources,
using char const * to specify uniform names.