Screensaver: Small style fixes to PreviewView
Screensaver: Convert Screensaver to a BDirectWindow

This allows OpenGL screensavers to be able to show a preview.
GLifeSaver: Show a preview

Also, we are safe to enable direct mode now.
Flurry: Show a preview

Also, we are safe to enable direct mode now.

This is untested due to this screensaver currently crashing but it should
work once #10206 is fixed.
Gravity: Show a preview

Also, we are safe to enable direct mode now.
Gravity: Style fix, rename msg parameter to message
Gravity: Live update settings and cleanup

Now that this screensaver shows a preview, update the settings live
so that the display updates immediately.

Since this didn't work before this screensaver took a shortcut and only
updated the settings when the saver started.

Some other changes include:
* Update copyright header, add myself to the authors list.
* Convert Particle from using a BList to a BObjectList and remove casts.
* Use switch statement to set particle color that has a default case for blue.
* Initialize member variables to a default value in the constructor.
* Remove Constants.h, the remaining constants are in ConfigView.cpp
* Convert deprecated GroupLayoutBuilder to a LayoutBuilder template instance
* Convert GravitySource from a class to a struct since all members are public.
* Simplify realCount calculation to just a single left shift.
* A bunch of style fixes
  - lots of whitespace fixes
  - rename rect to frame in GravityView and ConfigView
  - reorder the frame parameter first in GravityView and ConfigView
  - curly brace goes on same line as class or struct declaration

This turned out to be a bigger change than I originally intended to make.
Revert "Flurry: Show a preview"

This reverts commit 61935d56e6bd01fd58cb174969cd95bf3f0ab7e1.
Flurry: Show a preview... try number two

Also, we are safe to enable direct mode now (it already was)

This is untested due to this screensaver currently crashing but it should
work once #10206 is fixed.

Remove obvious comment in DirectConnected