[debugger] Xdebug not being detected during debugging [5.2.1-RC1]

In-Portal has built-in debugger that collects information about rendered page and then presents it upon request (by pressing F12 key on that page). When debugging using "Zend Studio Debugger" the built-in debugger is automatically turned off (based on presence of "debug_*" cookies).

However same isn't done, when debugging using XDebug. To detect XDebug we should check for XDEBUG_SESSION cookie as well.