Fix CursorInvoker wrongly positioning EventIcon palettes
Fix CursorInvoker wrongly positioning EventIcon palettes

Steps to reproduce:
1.  Long press on home view favorite icon (an event icon)
Expected:  Palette invoker where finger is
Actuall:   Palette invokes in top left corner

This is because the long press controller emits coords relative
to the widget's window, and EventIcons sometimes have their own
windows.  We can't change the long press controller, as it is a
public api.  But we fix this by translating the coords to the
root window coords in the CursorInvoker.  Using root coords is
consistent with the right click handleing in the cursor invoker.

i5o/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3