Use 'Windows-key' for opening main menu instead of 'Alt-F1' in Mate

- Type in terminal: mateconf-editor

- Go in the editor to:

   apps

   marco                           

   global_keybindings

- Change the value of 'panel_main_menu' from <Alt>F1 to Super_L

- From now you can use the Windows-key for opening your main menu