Revert "EditorManager: Fix crash when closing an editor upon activation"

Deleting an editor while currentEditorChanged is apparently not a good idea.
Use a queued connection instead.

This reverts commits 243a6259618e69b8da9ac2438480c449754f7f77 and
d9602ca550c53e60a5229bfec9539e41fa5a694c.

Task-number: QTCREATORBUG-10190
Task-number: QTCREATORBUG-10235
Change-Id: Iccbee25fb77714963faa6f7184d1f5f53ed348a6
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
Merge remote-tracking branch 'origin/2.8' into 3.0

Conflicts:
	src/plugins/coreplugin/coreplugin.pro
	src/plugins/coreplugin/coreplugin.qbs
	src/plugins/cppeditor/cppdoxygen_test.cpp
	src/plugins/cppeditor/cppquickfix_test.cpp
	src/plugins/cppeditor/followsymbol_switchmethoddecldef_test.cpp
	src/plugins/cpptools/cppcompletion_test.cpp
	src/plugins/qmldesigner/components/propertyeditor/propertyeditorwidget.h
	src/plugins/qmljseditor/qmljshighlighterfactory.cpp

Change-Id: Iba426e70b768df538393fc3cec96c7bff16a0658