Improve most used documentation pages

What is the benefit? Why is this valuable?

Make the most visited pages as high quality as possible

What are common use cases?

Users search or navigate to the page and search for information

Technical information

Review and improve following popular pages

• qstring
• qwidget
• qtdesigner-manual
• qcombobox (refresh images, \brief descriptions, link to example(s))
• stylesheet-reference (reorg unreadable tables)
• qml-qtquick-text
• qobject (massive inheritance table)
• qlineedit (needs code snippet(s), possibly screenshots)
• qfile (lots of overloads that can be combined into a single documentation block)
• qfiledialog (add image, check \brief descriptions, language and formatting)

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