Use Q_FALLTHROUGH to fix the build with gcc-6

Authored by heikobecker on Mar 4 2018, 12:50 PM.

Description

Use Q_FALLTHROUGH to fix the build with gcc-6

Requires Qt >= 5.8.0 but hides the complexity of when
"attribute ((fallthrough))" can be used (or what comments are
accepted to prevent the warning with gcc-7).
At least the latest Ubuntu (17.10), Fedora ( >= 26), Debian Buster,
Arch and Gentoo have at least 5.8.x, so it should hopefully be old
enough to depend on.