Add support for perfect-margin to work in the main buffer when a sidebar like org-side-tree is opened.

[IceAsteroid]

First of all, thank you @mpwang for the energy and effort spent in this package. It has become a must need for a wide monitor that I use daily!

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Perfect-margin seems to not keep working when a certain sidebar window is opened. Closing that sidebar would bring back effect of perfect-margin in the main buffer.

For those of you that may wonder, why not add a general solution to support various sidebar windows but to instead open an issue for a particular specific sidebar, see issue: #17

[mpwang]

Org-side-tree is a nice package and I'd like to use it too. Thanks for the suggestion.

After much effort I have rewritten the core logic to support using side window on the left and/or right side on the same time!

And also added the org-side-tree support, please try the latest and enjoy it.