use menu.prepend instead of menu.insert

The problem with "insert(..., 0)" is that on some(?) systems, the size of the menu is not adjusted automatically.
For example, on Linux Mint 17 Qiana (with Cinnamon as desktop environment) I can not really use the menu, because only one item is displayed at the same time and the menu has to be scrolled.
prepend() implements the same functionality but assures that the size of the menu is adjustic accordingly.

This is a neat way to run the unity-indicator in another desktop environment (namely Cinnamon).