Implementation of a list of open sketches in the Sketch menu

Here's how it has been implemented:  

* The getEditors() function in the Base class has been used to get a list of currently open editors when the user clicks on the Sketch menu, and the Open Sketches sub-menu is populated with the currently open sketch names, with a check mark on the current sketch.
* When the user clicks on a menu item, that skech is brought to the front.
* The Open Sketches sub-menu is de-popuated when the Sketch menu is deselected.