Fix for Bug #1252139 : Send Task via Email plugin - Add a button to the main window toolbar to send email

Fix for bug #1252139 : Send Task via Email plugin - Add a button to the main window toolbar to send tasks as email from Main window.

Added a button in main window, enabling users to send tasks as email from main window.
Can you merge it with onTbTaskButton? It is not good practice to duplicate exactly the same code