Remove old NVidia workaround from AbstractObject::getLabel().

This bug is fixed, but other issues appeared.
Use object at least once before testing its label.

Otherwise my NVidia emits GL_INVALID_VALUE with message "that <object>
is not valid object", which isn't in the specs. I hate this "deferred
everything" approach in GL. Is this even legal optimization?
Fix CubeMapTexture test.

This time thank you, hardened NVidia drivers.
Allocation-free overloads for *::setLabel().

In most cases the label is set directly from code, e.g.:

    texture.setLabel("diffuse-duck");

Avoiding conversion to std::string and passing char(&)[size] directly
will avoid one allocation and deallocation. Better solution would be to
use std::string_view everywhere, but we're not in C++17 yet.
Allocation-free overloads for AbstractShaderProgram string functions.

In nearly every case the attributes are bound and uniform locations
queried with constant char arrays:

     bindAttributeLocation("position", Position::Location);
     colorUniform = uniformLocation("color");

Avoiding conversion to std::string and passing const char(&)[size]
directly will avoid needless allocation (and later deallocation) for
every call.
TgaImporter: remove old nonsense from plugin configuration file.
Trade: allow instantiating importer plugins with access to manager.