Upgrade to Java 17, switch to Gradle

[dorving]

Currently, I have only tested the changes on MacOS. The runtime image creation for Windows still needs some work (specifically making a distinction between 32-bit and 64-bit images).

The following has been done:

• uses badass runtime plugin to create platform-specific images.
• switch from maven to gradle build script (kotlin)
• uses openjfx plugin to obtain the required javafx modules
• added G-Wasm jar to repo for now, maven link is broken for me
• added g_mem_mac executable (https://github.com/dorving/g_mem_mac)
• added MacOS readable icon (.icns format)
• removed unused and broken imports from PortRequester.java and GEarth.java
• added GEarthLauncher.java (see docs for reason)

This is still a work in progress!