Draft: swipearea: Add support for nested flickables

Fixes: #318

I've been unable to replicate the behavior in Flickable https://github.com/qt/qtdeclarative/blob/dev/src/quick/items/qquickflickable.cpp that allows for nested flickables to not conflict with each other, so I've pursued an uglier approach where I manually find whether receiving children have a Flickable parent, and determine if the flickable will be active.