Doc: Improve SQL Example: Drill Down Example

• Example application has wrong window title ('drilldown', should be 'Drill Down').
• Old logo in application and screenshot.
• Graphics in application has tearing/artifacts.
• Mixes first person plural and second person POV.
• The class descriptions should be reordered: first main application, then the other two.
• 'data base' -> 'database' (one word).
• Use of possessives, Latin expressions: rewrite to improve readability for non-native English readers.

See also:

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