ProjectManager: Make Open Project dialog open in default folder

Move open project dialog handling into ProjectExplorer and make it
use its list of project file patterns. Same in Checkout wizards.

Task-number: QTCREATORBUG-1759
Reviewed-by: dt

src/plugins/coreplugin/mainwindow.h

@@ -128,7 +128,6 @@ signals:
     void windowActivated();
 
 public slots:
-    void openProject();
     void newFile();
     void openFileWith();
     void exit();