[Qt] Move QMenu dependant scrollbar context menu handling out of WebCore

[Qt] Move QMenu dependant scrollbar context menu handling out of WebCore
https://bugs.webkit.org/show_bug.cgi?id=79233

Reviewed by Kenneth Rohde Christiansen.

Source/WebCore:

Move the code into WebKit/qt/Api/qwebpage.cpp, the only place where it is called from.

* Target.pri: Remove ScrollbarQt.cpp from build.
* platform/Scrollbar.h: Remove Qt-only context menu handling but make moveThum
accessible from the outside.
* platform/qt/ScrollbarQt.cpp: Removed. Not needed anymore.

Source/WebKit/qt:

Move scrollbar context menu handling here, it's the only place where it is
used.

* Api/qwebpage.cpp:
(handleScrollbarContextMenuEvent):
(QWebPage::swallowContextMenuEvent):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108498 268f45cc-cd09-0410-ab3c-d52691b4dbfc