Add clang-format style for the Apache Celix project

[pnoltes]

Currently Apache Celix has a mix of styles in its code base, ideally this should be align to a single and clear coding style.

A first step could be to introduce a clang-format style supported with some cmake / shell support to apply the clang-format.

For example, something like the following clang-format can be used as starting point:

#.clang-format
Language: Cpp
BasedOnStyle: Google
PointerAlignment: Left
DerivePointerAlignment: false
InsertBraces: true
AlignEscapedNewlines: Right
UseTab: Never
IndentWidth: 4

And a small shell command or cmake command can be created to apply the format to all Apache Celix c, h, cc and cpp files:

find bundles -iname "*.h" -exec clang-format -i {} \; 
find bundles -iname "*.c" -exec clang-format -i {} \; 
find bundles -iname "*.cc" -exec clang-format -i {} \; 
find bundles -iname "*.cpp" -exec clang-format -i {} \; 
find libs -iname "*.h" -exec clang-format -i {} \; 
find libs -iname "*.c" -exec clang-format -i {} \; 
find libs -iname "*.cc" -exec clang-format -i {} \; 
find libs -iname "*.cpp" -exec clang-format -i {} \; 

A shell command to only apply the format to git staged files can also be created, something like:

git diff --name-only --cached --diff-filter=d -- '*.h' | xargs -I {} clang-format -i {}
git diff --name-only --cached --diff-filter=d -- '*.c' | xargs -I {} clang-format -i {}
git diff --name-only --cached --diff-filter=d -- '*.cc' | xargs -I {} clang-format -i {}
git diff --name-only --cached --diff-filter=d -- '*.cpp' | xargs -I {} clang-format -i {}

The creating of the .clang-format file and the applying of format should be separate pull requests.