Integrated designer does not use correct extensions for promoted files

Creator allows the user to specify alternative extensions for common files (eg. .hpp instead of .h). However these are not picked up by the class-promotion editor in the designer which always assumes class header file names are lowercase and end with .h. This causes the build to fail if the user misses the extension applied.

The integrated designer should use the file extensions specified by the user in Creator's options dialog.

Fix for Qt 4.8.