[kstyle] use Q_FALLTHROUGH in breezesplitterproxy

Summary:
Qt has its own macro to tell the compiler that the lack of the break
statement is intentional. One of the advantages of using Q_FALLTHROUGH
is that it explicitly says other developers that the break statement is
missing on purpose. Another advantage is that we can use directives,
see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817

Test Plan:
Compiled Breeze with GCC 7 and Clang, no "this statement may fall
through" warnings whatsoever.

Reviewers: Breeze, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11860