introduce property for switching between bundle/non-bundle products

There should be a property to switch between bundle/non-bundle products.
If true, it should build bundles on OS X / iOS and be ignored everywhere else.

That's to avoid conditionals in user code for the product type.

Proposal:

Application {
    createBundle: true   // will create app bundle on OS X
}
DynamicLibrary {
    createBundle: true   // will create framework bundle on OS X
}