Retire native launcher code

[ctrueden]

We want to evaluate whether it's possible to have a pure Java launcher.

The native portion would be some stock launcher such as the JavaFX launcher (see #33), or the newer experimental jpackage tool.

Probably, once Java starts up, it would need to find a suitable JDK on the system to use for actually launching the main application, and "spawn Java from Java." This gives us the power to do things like:

• Launch Python which then spins up a JVM—so that CPython stuff can be used from ImageJ.
• Reconfigure the JVM's memory settings, garbage collector flags, etc., really any custom JVM-level flag can be given on the CLI and it will be propagated to the secondarily spawned Java.
• XInitThreads library needs loading very early in the application lifecycle (see #64)—probably before any AWT classes are loaded? It might work to hard-code this into the Java-side launcher code even before the splash screen (see next paragraph) is shown. But testing needed.

Regarding splash screens: the current launcher code here now uses a pure-Java splash mechanism with minimal class loading, which is fast enough and also cross platform.

[ctrueden]

The Java launcher needs to know:

• What is the minimum required version of Java? The launcher's version of Java (e.g. Java 6 or 8?) might be unsuitable for actually running full-blown ImageJ (which we'd like to update to require Java 11+).
• Does it need to be a JDK? Or is a JRE good enough? For ImageJ, a JDK is preferred because the Script Editor can run Java code only when a Java compiler is available.

And ideally, the Java launcher would be able to move staged files (from the update folder) into place before launching the appropriate Java.

[ctrueden]

We are switching the Fiji launcher to a new one we made called Jaunch. It is much more flexible than the built-in Java jpackage standalone app bundling tool.

Jaunch does not quite eliminate all native code, but it does drastically reduce the C footprint in favor of using Kotlin for most of the heavy lifting.