Menu cleanup

We had a lot of stuff in the menus that was not really needed and some other stuff that was poorly organized. Here is a summary:

* Removed the Kernel menu entirely. All kernel actions are in the toolbar.
* Added a "Restart Kernel" button to the toolbar.
* Removed the cell selection menu items from the "Cell" menu. The are again the toolbar.
* Cleaned up the output management menu items in the "Cell" menu.
* Removed the unneeded "Select Prev|Next" from the "Edit" menu.

Yeah simplification!