Allow SImageWriter to handle timestamps and bitmap images

Description

• Handle timestamps in SImageWriter via a slot (implemented in IWriter). If specified it will be appended as a prefix when an image is written. This will allow to save images from a timeline without having to change the basename at each step (notably when using oneShot mode).
• Add bitmap image support for SImageWriter

Proposal

• Add a new slot for handling timestamps in IWriter
• Implement BitmapImageWriter to write bitmap images using VTK

Context

Those features will be first used in USTools in the imageFromUSCTRegistration activity that allows to synchronize an US image tracked via electromagnetism and an image extracted from the CT. The US images are read via timeline from files named after timestamps. The images from CT are generated in the application. The aim here is to assign the same timestamp in the filename for each corresponding pair of US image and CT extracted image