mac-capture: Improve various Objective-C code

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Improve complicated Objective-C implementations in
mac-screen-capture implementation.

- Use `for` instead of overuse of block based iteration code.
- Use `YES` and `NO` for `BOOL` type instead of wrong `TRUE`, `FALSE`,
  `true`, `false`.
- Use `length` to check empty string, omit unnecessary `nil` checks.

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