[Qt] Add developerExtrasEnabled to QWebPreferences.

[Qt] Add developerExtrasEnabled to QWebPreferences.
https://bugs.webkit.org/show_bug.cgi?id=83018

Reviewed by Kenneth Rohde Christiansen.

This is required to allow inspecting pages.

* UIProcess/API/qt/qwebpreferences.cpp:
(QWebPreferencesPrivate::testAttribute):
(QWebPreferencesPrivate::setAttribute):
(QWebPreferences::developerExtrasEnabled):
(QWebPreferences::setDeveloperExtrasEnabled):
* UIProcess/API/qt/qwebpreferences_p.h:
* UIProcess/API/qt/qwebpreferences_p_p.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@113022 268f45cc-cd09-0410-ab3c-d52691b4dbfc