Added "Enable Dash Platform Detection" menu item and some "usability" fixes

I've had users confused by the "Open Quick Help in Dash" menu item (they were clicking it trying to trigger the Dash search...).

I did the following:
1. Added a "Dash Integration" menu item that has a submenu
2. Moved the "Open Quick Help in Dash" menu item into the Dash submenu and renamed it to "Enable Dash Integration" just to make it clear
3. Added a "Enable Dash Platform Detection" menu item to enable/disable the automatic platform detection

I've also changed the README a bit to make it clear that users have to freakin' **Option+Click** stuff.