Open a mirrored NERDTree with correct width

[PhilRunninger]

Description of Changes

Closes #1176. {lib/nerdtree/creator.vim}

When mirroring a NERDTree buffer on another tab, it was being displayed with the default width, even if the original NERDTree window was zoomed (with the Shift+A key). This PR fixes that issue, by restoring the window's size and position after displaying the NERDTree buffer in the tab's new window.

To recreate the issue:

:let NERDTreeQuitOnOpen=1
:NERDTree
    Shift+A   - to zoom the NERDTree window
    t         - to open a file in a new tab
:NERDTreeMirror    - to open the same NERDTree buffer in the new tab

There was some cleanup done in the PR as well.

1. An unused variable was removed; it was referenced, but never set anywhere. {lib/nerdtree/opener.vim, lib/nerdtree/ui.vim}
2. The variable initialization code was updated to use the get() function for more direct assignment of values. {plugin/NERD_tree.vim}

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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