Fix missing mode changed signal on startup

Reverts commit 88fc878aa8
and fixes the original issue differently.
Introduce a currentAboutToChange signal in FancyTabBar that
is passed on via FancyTabWidget to
ModeManager::currentModeAboutToChange.
This avoids the temporary mismatch of selected indices in the
tab bar vs tab widget.

Change-Id: If1a331db5478450d7c57e5908b2bed218e9828a8
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/coreplugin/fancytabwidget.h

@@ -125,6 +125,7 @@ public:
     QRect tabRect(int index) const;
 
 signals:
+    void currentAboutToChange(int index);
     void currentChanged(int index);
     void menuTriggered(int index, QMouseEvent *event);