[rfc][icedtea-web] Fix for PR1251: AppContext has wrong context classloader

[Adam Domurad]

This fixes one of the (numerous) issues in ADOMII, and fixes SweetHome3D 
furniture dragging.

I'll preface by saying, I'm not particularly fond of the changes in this 
patch, but they have to be done to fix JNLP applications that load 
resources on the Swing thread, like eg SweetHome3D.

The patch adds a flag to JNLPClassLoader that specifies whether it has 
been initialized. The default for getInstance remains the same, it grabs 
an initialized JNLPClassLoader. However an alternate getInstance was 
added to be able to grab JNLPClassLoader that hasn't been fully initialized.

The problem with initialization:
     - To create AppContext we need JNLPClassLoader object constructed
     - To construct JNLPClassLoader we must initialize it
     - Initializing JNLPClassLoader implies downloading all resources, 
and updating Swing components

However, we cannot update swing components before we create our 
AppContext, or the result will be a mess (eg unclickable popups).
In essence, it's a bootstrapping problem. So the patch makes it possible 
to get a constructed JNLPClassLoader, which is initialized in a separate 
step.

What else was tried ?
     The only other option is to set the context classloader to some 
other classloader. A proxy classloader can be created that initially 
points to the system classloader, but can be set to JNLPClassLoader 
after it is created. Determined too much of a hack, and getParent cannot 
be properly proxied.

Basically with this patch, the life-time of the context classloaders for 
each thread is:
    1. By default system classloader is context classloader.
    2. Create new AppContext, capture JNLPClassLoader that isn't yet 
valid. Ensure we keep using system classloader as context classloader 
for all threads.
    3. Initialize JNLPClassLoader. Before start of applet, ensure all 
threads context classloaders are set to (now initialized) JNLPClassLoader.

Recommendations welcome -- it was a lot of deliberation before I decided 
to take this route. If someone has a simpler/cleaner solution than by 
all means suggest it!

Reproducer changelog:
2013-XX-XX Adam Domurad  <adomurad at redhat.com>

     * 
tests/reproducers/signed/AppContextHasJNLPClassLoader/resources/AppContextHasJNLPClassLoader.html:
     Test AppContext context classloader from HTML applet
     * 
tests/reproducers/signed/AppContextHasJNLPClassLoader/resources/AppContextHasJNLPClassLoader.jnlp:
     Test AppContext context classloader from JNLP application
     * 
tests/reproducers/signed/AppContextHasJNLPClassLoader/resources/AppContextHasJNLPClassLoaderForJNLPApplet.jnlp:
     Test AppContext context classloader from JNLP applet
     * 
tests/reproducers/signed/AppContextHasJNLPClassLoader/srcs/AppContextHasJNLPClassLoader.java:
     Print out context classloader for thread & AppContext, for
     current thread & Swing thread.
     * 
tests/reproducers/signed/AppContextHasJNLPClassLoader/testcases/AppContextHasJNLPClassLoaderTest.java:
     Test runner for AppContextHasJNLPClassLoader

REPRODUCER NOTES:
  This exposes an additional bug in JNLP applets. start/init incorrectly 
runs on the Swing thread, and invokeAndWait hangs forever. I'm not sure 
how this wasn't caught, but the JNLP applet reproducer is broken pending 
a separate bug fix.

Fixes changelog:
2013-XX-XX Adam Domurad  <adomurad at redhat.com>

     Fix PR1251: AppContext has wrong context classloader
     * netx/net/sourceforge/jnlp/Launcher.java:
     Remove unused context flag. Make new AppContext capture not-yet-
     initialized JNLPClassLoader.
     * netx/net/sourceforge/jnlp/NetxPanel.java:
     Move PluginBridge initialization into constructor.
     Make new AppContext capture not-yet-initialized JNLPClassLoader.
     * netx/net/sourceforge/jnlp/runtime/JNLPClassLoader.java:
     Add initialization flag. Move part of constructor into initialize
     method. Modify getInstance to allow temporarily bypassing
     initialization.
     * netx/net/sourceforge/jnlp/runtime/JNLPRuntime.java
     (createAppContext): New, creates an AppContext that captures the given
     (not necessarily yet valid)

-Adam
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