Free42 state file switching, export, and import

[Thomas Okken]

The test builds had a nasty bug that would cause programs to become corrupted. I fixed it and uploaded new builds for all platforms.

If you weren't using the test builds, you're OK, because the production builds don't have this bug. If you were or are using any of the test builds, you should either go back to using production builds, or upgrade to the very latest test builds, and reload all your programs from a known-good source. That last part is important because the corruption may not be immediately apparent.

N.B. Some of you may have noticed an Android build among the test builds; I hadn't advertised its presence yet because there is still work to be done there. It works great in the Android Emulator, but on my real phone (Moto G5), loading *.f42 (state) files from email attachments and web servers is still not working, for as yet undetermined reasons. Apart from that flaw, it's no worse than the others.