Commit 37fa0d19 replaced gtk_menu_popup with gtk_menu_popup_at_rect
to avoid deprecated warning making it more complicated then needed.

gtk_menu_popup_at_rect requires two parameters that are not always
available. For example we don't have GdkWindow for CSD windows. And
next commit will popup menu in response of ClientMessage event where
we don't have any info about key and/or button press events.

Simplify code by creating fake event in menu.c with minimal data
needed for gtk_menu_popup_at_rect. GdkEvent is used to get GdkDevice,
button and time. For GdkWindow we can use root window.