Core::Constants: Rename close icon constants to reflect file name

src/plugins/coreplugin/coreconstants.h

src/plugins/coreplugin/editormanager/openeditorsview.cpp

src/plugins/coreplugin/editortoolbar.cpp

src/plugins/coreplugin/find/findtoolbar.cpp

src/plugins/coreplugin/manhattanstyle.cpp

src/plugins/coreplugin/navigationsubwidget.cpp

src/plugins/coreplugin/outputpanemanager.cpp

src/plugins/coreplugin/sidebarwidget.cpp

src/plugins/help/helpplugin.cpp

src/plugins/help/helpwidget.cpp

src/plugins/help/openpageswidget.cpp

src/plugins/projectexplorer/buildstepspage.cpp

...gins/qmldesigner/components/importmanager/importlabel.cpp