feat: window capture (Mac)

[minkyu97]

Changes proposed in this PR

• Fixes Feature: capture background window #331
• I've spent a long time deciding which fields should be included in Window type as all of those fields should be implemented on other platforms as well. Finally, I found that, at least on Windows, window name, process name, and window bounds can be implemented easily. If you have any different opinions, please feel free to share them.
• Even though there are some tests for my implementation, I think they aren't sufficient. I'm not sure how to properly verify my codes, as the results directly depend on current windows and processes. I would appreciate any suggestions on better approaches.
• This PR includes only the OSX implementation, and additional development is needed for Windows and Linux.

It is very important to keep up to date tests and documentation.

• Tests added/updated
• Documentation updated

Is your code right?

• PEP8 compliant
• flake8 passed