Make it clearer where the user should enter the custom start page
When the user chooses to use a custom start page, he may not realize where he's supposed to enter it and may think that it's the "Home page" widget (also because most browsers don't allow to customize the home page and the starting page separately).
To improve this situation, given that adding a label to the widget would take too much space, the following changes have been implemented:
- only display the custom start page widget when the user actually chooses to use a custom start page
- add a message widget warning the user if he leaves the widget empty.