[albireo-dev] More focus related fixes and some cleanup

[Gordon Hirsch <gordon.hirsch@xxxxxxx>]

I've fixed two more focus related issues.

1) The embedded frame and the JApplet use the default AWT focus 
traversal policy. To my surprise I discovered that this default can 
change from LayoutFocusTraversalPolicy (set by the Windows L&F) to 
LegacyGlueFocusTraversalPolicy if javax.swing.FocusManager.getManager() 
is called. To my greater surprise I found that this swing method is 
called by the Eclipse runtime itself (see reflection in 
org.eclipse.ui.internal.handlers.WidgetMethodHandler). The legacy focus 
traversal policy does not work well in our environment and leads to 
stack overflow errors when a swing component is focused. So, to avoid 
the problem I force the focus traversal policy (on both the frame and 
the JApplet) to LayoutFocusTraversalPolicy. Windows only. I don't know 
if the problem exists on other platforms.

2) If an app tries to set focus on a SwingControl, and the embedded 
Swing control is not focusable, the SwingControl should refuse the focus.

I also did a little cleanup. I removed the SwingControl.checkPopulated 
method since it is no longer necessary. I added checkWidget() calls to 
all public API in SwingControl except for the accessors for the frame 
and swingComponent.