Cleanup help configuration page.

- Expand the 'home page' field in help dialog: it used to be quite small, with some space
left on the same line.
- Align labels them with the middle of the combo (center vertically).

Change-Id: Ife1f73e25d7c1aa7ca7e82782c6858e19caad4a9
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>