PR: Improve PathBoxWidget and add option to use a QToolButton for 'Browse'

[jnsebgosselin]

This pull request introduces several improvements to the PathBoxWidget component in qtapputils:

• Adds support for specifying a custom icon for the Browse button. When an icon is provided, the widget uses a QToolButton; otherwise, it defaults to a QPushButton.
• Refactors and cleans up the internal codebase, improving maintainability and readability.
• Enhances flexibility for developers integrating PathBoxWidget into their applications.

These changes make it easier to customize the widget’s appearance and integrate it seamlessly with different UI designs.