Add new GulkanVertexBuffer and GulkanDescriptorSet

This series of patches introduces a new vertex buffer API that makes it possible to initialize an arbitrary number of attributes with offsets in one VkBuffer in favor of the hard coded ones available before.

Furthermore the GulkanDescriptorSet class abstracts the way we initialized descriptor sets, with improvements in GulkanUniformBuffer and GulkanTexture to generate descriptors. The GulkanTexture is now able to optionally initialize and hold a VkSampler.

The patch set ports the examples to the new API and cleans them up a bit, especially in terms of duplication by removing the model-renderer example classes.