Doc: Improve SQL Example: SQL Browser

• Table overview pane does not default to full content width (adds scrollbar).
• Resizing the id column to a sensible size makes the example more visually appealing (and solves the point above).
• The database pane title is lower-case.
• Clicking Submit with an empty SQL Query returns "Query OK, number of affected rows: -1" as status. This is not informative nor intuitive. The Clear and Submit buttons should be inactive until they actually do something.

See also:

• https://wiki.qt.io/Examples_Revamping_Qt_SQL#Notes
• https://wiki.qt.io/Examples_Revamping_Qt_SQL#Examples