JsonWizard: Allow the summary page to change to the file list

Change the JsonWizard interface:
* generateFileList now does no longer change the state of the JsonWizard
* commitToFileList added which is used to commit the wizard to a final
  list of files.
* Trigger generateFileList when version control is changed in the
  SummaryPage
* Commit to the file list once the SummaryPage is done

Change-Id: Ieb7b7abbf428d96596526c01946ecf0852f65744
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
JsonWizard: Add .gitignore file to empty qmake projects

Task-number: QTCREATORBUG-3125
Change-Id: I8c6777734224d4f7fff27ce441f75f380460723a
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>