Make image usage flags for swap chain images configurable

[johannesugb]

...because this:

mImageUsage = avk::image_usage::color_attachment | avk::image_usage::transfer_destination | avk::image_usage::transfer_source | avk::image_usage::presentable;
const vk::ImageUsageFlags swapChainImageUsageVk = vk::ImageUsageFlagBits::eColorAttachment | vk::ImageUsageFlagBits::eTransferDst | vk::ImageUsageFlagBits::eTransferSrc;

is not cool. See window.cpp#L591!