Use gcc -Wimplicit-fallthrough=2 by default if available

Use gcc -Wimplicit-fallthrough=2 by default if available

GCC 7 instroduced -Wimplicit-fallthrough
https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/

It caught a couple of bugs, but it does need a bit of extra comments to
explain when a switch case statement fall-through is deliberate. Luckily
with -Wimplicit-fallthrough=2 various existing comments already do that.
I have fixed the bugs, but adding explicit break statements where
necessary and added comments where the fall-through was correct.

https://bugs.kde.org/show_bug.cgi?id=405430