sigrok AppImage updates

As you probably know, we've been providing pre-built Linux binaries for PulseView and sigrok-cli in the form of AppImages for a while now. Those can be useful/convenient if you want to test sigrok without having to build from source, and/or if the packages of your favorite Linux distro are out of date.

You can simply download a single *.AppImage file, make it executable via "chmod+x" and run it (see the wiki for details).

As of today, the AppImages we provide are now being built on Ubuntu 16.04 LTS Xenial Xerus (originally released 2016) instead of the previously used Ubuntu 14.04 LTS Trusty Tahr (originally released 2014, Canonical support ended 04/2019).

This has been done since we're now using a more recent version of Qt for PulseView (that is not easily available in the older Ubuntu version), and also to get more recent versions of various other libraries we use in general.

This change shouldn't affect too many people, the new AppImages should work fine on pretty much any modern Linux distro which is somewhat newer than Ubuntu 16.04 LTS Xenial Xerus from 2016.