ProjectWizardPage: Refactor code a little

Move functionality from the projectfilewizardextension into the
projectwizardpage itself and reduce the size of the context shared
between the extension and the page to a bare minimum.

Change-Id: I785fc103ea7d12e1da4bfdaf1d4664ed83a9989b
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
ProjectWizardPage: Move code populating the Project dropdown

... from the ProjectWizardExtension into the ProjectWizardPage.
It is easier to reuse that way.

Change-Id: Id9d32735cb695358a9bc283bc7545ecb28c4e04e
Reviewed-by: Daniel Teske <daniel.teske@digia.com>