Add 'quick save image' menu option
Allow to save RGB48 data as 16-bit PNG

As AviSynth doesn't support 16-bit depth, each frame of the clip
returned by the script is assumed to be RGB48 data with a real
height half the reported by AviSynth. This can be achieved with
the following (Dither package):

Dither_convert_yuv_to_rgb (lsb_in=true, output="rgb48yv12")

or

Dither_convey_rgb48_on_yv12 (r, g, b)

As of wxPython 2.8 (stable) and 2.9.4 (development), additional
options can't be shown still in a 'save file' dialog. So, for now
instead of asking if the clip is 16-bit like for the JPEG quality
the script is parsed for one of the function calls above.

Added also a 'depth' parameter to the SaveImage macro function.
Add setting for not prompting for JPEG quality

The last value introduced in the prompt is used, 70 as default.
Update 'Save Image Sequence' macro to make use of the previous changes
Notify on the status bar when an image is 'quick-saved'