Fix context of Locator input

- In the main window, the locator input (actually the status bar)
  visually feels like being part of the mode widget, so give
  it its context.
- In extra editor windows, the whole editor window should have
  "editor manager" context, so that is also active for the locator
  input.

Task-number: QTCREATORBUG-20626
Task-number: QTCREATORBUG-20071
Change-Id: Ib68d6a8177446572ea59c3cc057eca0706173e11
Reviewed-by: Xing Xiong
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/coreplugin/icore.h

@@ -112,6 +112,7 @@ public:
 
     static IContext *currentContextObject();
     static QWidget *currentContextWidget();
+    static IContext *contextObject(QWidget *widget);
     // Adds and removes additional active contexts, these contexts are appended
     // to the currently active contexts.
     static void updateAdditionalContexts(const Context &remove, const Context &add,