Apply modern formatting tools

The C++ code in kmix is pretty mixed. For instance, in the mixer_oss.cpp file, I can see 2-space, 4-space, tabs, tabs-and-spaces, and no indentation. This branch adds an .editorconfig and .clang-format (copied from other KDE repositories) in an attempt to standardise the formatting of the C++ code.

There's one commit showing what the effect is; I didn't want to hammer the entire codebase at once.