Don't restrict org outline path length

[mosquito-magnet]

This tries to address two issues by removing restriction of outline path length to window width in helm-org-agenda-files-headings:

1. Truncating the outline paths seems counterproductive.
2. Bug: The org outline path candidates for helm-org-agenda-files-headings get truncated to the width of the previously active window, not the helm-window.

Reasoning for 1.:
The leaf node is likely the most interesting part of the candidate, it shouldn't be cut off. In comparable scenarios like helm-find-files there is also no truncation.

Not truncating candidates will use the full helm window width, and users can use horizontal scrolling or wrapping (via helm-org-truncate-lines or helm-toggle-truncate-line) to inspect the full path if required.

Repro steps for 2.:

• Start emacs, split window vertically
• Open some org file and create entries that result in outline paths longer than the current window width
• Ensure that org file is part of org-agenda-files
• Call helm-org-agenda-files-headings
• The helm candidate window should open below the 2 existing vertically split windows, using the full frame width
• The candidates are truncated to match the org window width, not the helm window width

The reason seems to be that helm-window returns nil when helm-org--get-candidates-in-file is called, so the window-width call returns the width of the current window instead.

Behaviour seems similar to the now inaccessible issue 1294 in helm, if that is of any relevance. Still available currently via google webcache:
https://webcache.googleusercontent.com/search?q=cache:BPTMko9B-8kJ:https://github.com/emacs-helm/helm/issues/1294

• Emacs versions tested with:
  GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo version 1.15.10) of 2020-09-19
• Org versions tested with:
  Org version 9.4 from package org-plus-contrib version 20201005
• helm versions tested with:
  3.6.2
• helm-core versions tested with:
  20200905.1311