BaseFileWizard: Rename selectExtensions method

Rename selectExtensions method of BaseFileWizard to extensions and
fix the fallout.

Mark the method as const while at it.

Change-Id: I716988fec464e06df6b128e4b3754164e61cd7a1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
BaseCheckoutWizard: Clean up a bit

Change-Id: I7ae3a18f52f946bf8a025fd3b8c1425bb2d88272
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>