Begin preferences purge

(This is going to be an ongoing project.)

When I first created a full-featured preferences handler, it was
tempting to dump a whole bunch of options there just because I could.
Now that I'm older and wiser (?), I realize that I have frequently used
preferences as a way to avoid making hard design decisions.

Slowly but surely, I hope to remove a number of superfluous preferences
from the program.  This commit removes a few dumb interface preferences
that should always be enabled (remember window location and dynamic
taskbar icons).
By default, assign new image forms the main PD icon

This icon is shortly replaced by a thumbnail of the loaded image, but
for images that take a long time to load (e.g. RAW images), the user may
catch a glimpse of the crappy old VB icon before the image thumbnail
appears.  This should remedy that.