Fix split resizing during zoom toggling

[ds2606]

Invoking NERDTree zooming with 'A' resizes the window to the full screen
width. However, on unzooming, vertical splits are left unequalized, with
one split generally occupying the remainder of the screen (leaving the
other splits with zero-width).

This commit automatically equalizes vertical splits upon un-zooming
NERDTree.

Description of Changes

Closes #1302

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New Version Info

Author's Instructions

• Derive a new MAJOR.MINOR.PATCH version number. Increment the:
  • MAJOR version when you make incompatible API changes
  • MINOR version when you add functionality in a backwards-compatible manner
  • PATCH version when you make backwards-compatible bug fixes
• Update CHANGELOG.md, following the established pattern.

Collaborator's Instructions

• Review CHANGELOG.md, suggesting a different version number if necessary.
• After merging, tag the commit using these (Mac-compatible) bash commands:

  git checkout master
  git pull
  sed -n "$(grep -n -m2 '####' CHANGELOG.md | cut -f1 -d: | sed 'N;s/\n/,/')p" CHANGELOG.md | sed '$d'
  git tag -a $(read -p "Tag Name: " tag;echo $tag) -m"$(git show --quiet --pretty=%s)";git push origin --tags

[rzvxa]

hey @ds2606, Thanks for the contribution, this code has been merged in #1395.

Have a great day!