[qtc] Adapt fix to QTCREATORBUG-10891

Original patch by Kai Koehne <kai.koehne@digia.com>,
commit acca83add48df2aea9dd29507083152ac1b52f0a

    Fix crash on exit

    Fixes a crash on exit in ~AnalyzerManager, where
    EditorManager::instance() might be already 0.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
[qtc] Additional fixes for bug 10891

Original patch by Kai Koehne <kai.koehne@digia.com>,
commit 601951a4e837f02f63ed6a5c2e199db9d325ccc0

    Clean up EditorManagerPlaceHolder

    The fix in acca83add48df wasn't enough: One has to make sure that
    even ~EditorManagerPlaceholder doesn't delete EditorManager implicitly,
    since it's explicitly deleted in ~MainWindow.

    Anyway, EditorManagerPlaceHolder::current() is never used, so we
    can get rid of this static variable entirely and simplify the code
    by just checking for EditorManager::parent() in the PlaceHolder
    destructor.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
Merge branch 'qtbug10891'