Automatically import plugins in all applications with static Qt

Automatically import plugins in all applications with static Qt

Since all gui applications already need some QPA plugin added,
we might as well add the default plugin and generate the code
to import the plugins automatically.

User can opt out from the automation by removing relevant
items from CONFIG variable: link_qpa_plugin or import_plugins.

Task-number: QTBUG-28131
Change-Id: Ic171c363464c099143374d3e39bcc28f6edf73d2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>