specializedprojectors: shorten package name

We can call it specialized, since it is a subpackage of
net.imglib2.projectors already.
compositeprojectors: shorten package name

We can call it composite, since it is a subpackage of
net.imglib2.projectors already.
volatileprojectors: shorten package name

We can call it volatiles, since it is a subpackage of
net.imglib2.projectors already. (We cannot call it
"volatile" because it is a reserved Java keyword.)
Volatile: migrate to base net.imglib2 package

The class is used by both projectors and converters,
so belongs in neither subpackage.
volatileconverters: shorten package name

We can call it volatiles, since it is a subpackage of
net.imglib2.converters already. (We cannot call it
"volatile" because it is a reserved Java keyword.)
Adjust ScreenImage package

We now call it net.imglib2.display.screenimage. The singular tense
matches most other package names. We also migrate it out of the
projectors subtree, because ScreenImages are not Projectors
specifically.
Use singular tense for net.imglib2.projector package

This is more consistent with the rest of the core ImgLib2 package names.
Generalize ScreenImage interface

It is now generic typed, so that it is possible to code ScreenImage
implementations that produces images other than AWT ones. There is
now a net.imglib2.display.screenimage.awt subpackage for the
AWT-based ScreenImage implementations.