Provide clang-format target with a KDE Frameworks style file

Summary:
Provides a clang-format target if wanted

Example usage:

include(KDEClangFormat)

add clang-format target for all our real source files

file(GLOB_RECURSE ALL_CLANG_FORMAT_SOURCE_FILES autotests/src/*.cpp autotests/src/*.h src/*.cpp src/*.h templates/*.cpp templates/*.h)
kde_clang_format(${ALL_CLANG_FORMAT_SOURCE_FILES})

Test Plan: Tried that above usage thingy in KTextEditor

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: zzag, sitter, mwolff, ochurlaud, nalvarez, kossebau, aacid, davidedmundson, dhaumann, apol, ognarb, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D24568