When navigating the roster items of an account with keyboard up/down, it is handy if the keyboard menu button (Gdk.KEY_Menu) pops up the context menu of the contact item currently under focus. This MR implements this functionality.
One issue is, I had to make a dummy Gdk.EventButton that can be passed on to
_show_contact_menu and in turn, to
GajimPopover() due to
Gdk.EventButton as a parameter.