Fix updating QgsApplication::libraryPaths; load before any Qt plugin use

[dakcarto]

Previously, it was happening after setting the default window icon.

For macOS, refactor libraryPaths update to ensure both QT_PLUGIN_PATH
and qt.conf are properly honored (fixes 4.5 year old bug), with
prioritization of qgis libs or libs shipped with .app bundle.

---

Description

Include a few sentences describing the overall goals for this PR (pull request). If applicable also add screenshots.

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and containt sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[dakcarto]

@m-kuhn, @nyalldawson I've no unit tests here because the code is in main.cpp. Is there a way to test that currently?

Conversely, I think these updates can be moved to the end of QgsApplication::init(); then, it can be properly tested. Opinions?