Callbacks: exec invoke & modal now use a typed enum wmOperatorStatus.
This helps avoid mistakes returning incompatible booleans or other
values which don't make sense for operators to return.
It also makes it more obvious functions in the WM API are intended
to be used to calculate return values for operator callbacks.
Operator enums have been moved into DNA_windowmanager_enums.h
so this can be used in other headers without loading other includes
indirectly.
No functional changes expected.
Ref !136227
