GENERATED prop: Add implementation for policy CMP0118 being set to NEW

* Adding implementation for policy CMP0118 being set to `NEW`.
* Adding new tests for policy CMP0118 being set to `NEW`.
* Checking the `GENERATED` property with `get_source_file_property` or
  `get_property` now always returns exactly `1` or `0`. No other values
  will be returned. Note, that this is a backwards-incompatible change,
  even when policy CMP0118 is unset or set to `OLD`.
* Additionally, as `get_source_file_property` and `get_property` now
  always check if a source-file was marked globally visible, even when
  CMP0118 is unset or set to `OLD`, they possibly return `1` where they
  might have returned `0` before the changes introduced by this commit.
  Note, that this is a backwards-incompatible change, even when policy
  CMP0118 is unset or set to `OLD`.
* As a consequence, the tests for policy CMP0118 being unset or set to
  `OLD` got slightly adjusted, too, to reflect these changes in
  behavior.