Create Snap packaging for Jalview (complete package for various Linux distros, primarily Ubuntu)

Snap is an all-in-one packaging for applications running on Linux. It is championed by Canonical Inc so features particularly in Ubuntu (alongside more usuall .deb files).

Snap packages are self contained so all dependencies are shipped in the package. This fits in with packaging the correct JRE for Jalview to run.

See https://snapcraft.io/ and in particular:
https://snapcraft.io/docs/java-applications
https://snapcraft.io/docs/gradle-plugin

Snaps also auto-update (checking four times a day) so, like the planned .deb and .rpm versions of Jalview this packaging should not include getdown, or at least not with its updating feature active.

If not using install4j and getdown then an alternative mechanism for setting file associations, opening file association clicks, jvl files, and memory settings should be devised.