Add Mesh::id() for consistency.
Xlib.h, I hate you sincerely.
Minor cleanup.
Added EXT_debug_label extension to the list.
external: updated glLoadGen-generated GL headers.

Added EXT_debug_label to them.
Remove obsolete glLoadGen-related workarounds.
Support for object labels from KHR_debug and EXT_debug_label.

Can't test EXT_debug_label, as that is apparently OSX 10.9-only. Added
GL tests for all implemented objects. KHR_debug is selected first, if
that is not available, fall back to EXT_debug_label. If neither is
available, the functions are no-op.

I hope EXT_debug_label gets replaced by KHR_debug later, thus it is now
only "emulated" through KHR_debug enums.
Platform: display label limits in magnum-info.