Core: Icon-only option for the mode selection bar

This adds an icon-only mode to the mode selection bar (and action bar).
A newly introduced Action can cycle between icon+text, icon-only and
hidden.
Also, the "Window" Application menu gets a submenu with the three
styles.

Task-number: QTCREATORBUG-18845
Change-Id: I4e0c453f6d920dfbfd795b8b054f6ff392a8700a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/modemanager.h

@@ -46,6 +46,12 @@ class CORE_EXPORT ModeManager : public QObject
     Q_OBJECT
 
 public:
+    enum class Style {
+        IconsAndText,
+        IconsOnly,
+        Hidden
+    };
+
     static ModeManager *instance();
 
     static Id currentMode();
@@ -55,10 +61,11 @@ public:
 
     static void activateMode(Id id);
     static void setFocusToCurrentMode();
-    static bool isModeSelectorVisible();
+    static enum Style modeStyle();
 
 public slots:
-    static void setModeSelectorVisible(bool visible);
+    static void setModeStyle(enum Style layout);
+    static void cycleModeStyle();
 
 signals:
     void currentModeAboutToChange(Core::Id mode);