Use std::string::data() instead of std::string::c_str().

Meaningless changes on meaningless day.
Remove offensive comment.

EXT_debug_label originated in ES and that was way before KHR_debug. In
fact, KHR_debug is inspired after that.
modules: updated FindCorrade.cmake from Corrade repository.
Use CORRADE_TARGET_WINDOWS instead of _WIN32.
Minor cleanup.
Added EXT_debug_marker extension to the list and GL headers.
New class DebugMessage for inserting debug messages via KHR_debug.

Previously this was done in DebugMarker only via GREMEDY_string_marker
on desktop and EXT_debug_marker on ES, now supporting both three on
desktop and KHR/EXT version on ES. The old DebugMarker is now only a
thin wrapper around DebugMessage, is marked as deprecated and will be
removed in future release.
Platform: display DebugMessage limits in magnum-info.
Disallow overriding exec() in GL tests.
Support for debug message callback from KHR_debug.