feat(linux) add with_temp_icon_dir builder extension

[Beanow]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

This is the replacement approach for #450.
Rather than detecting Flatpaks with Tao, we'll allow setting a temp_icon_dir to override default behavior.

Note, some refactoring was involved so I've made several commits for easier review.

Also as this adds a new (non breaking) feature, I assumed this is a minor version increment.

Testing:

• The unit test is updated.
  1. Run with cargo test --features=tray.
• The example is updated.
  1. Run with cargo run --features=tray --example=system_tray.
  2. Verify that the example created an icon at path /tmp/tao-examples/*.png