which context to run the operator: WM_OP_DEFAULT, WM_OP_REGION_WIN,
WM_OP_AREA or WM_OP_SCREEN. This also replaces WM_operator_call_rwin
since it is more general.
This is useful for buttons and popup menus to run operators, and also
used by a new function to lookup the keymap item for that operator in
the right context.