code: Add clang-format target and CI workflow (#82)

* code: Add clang format target, rules and CI workflow

* code: Run clang format on sources

.ci/clang-format.sh

@@ -0,0 +1,37 @@
+#!/bin/bash -ex
+
+if grep -nrI '\s$' src *.yml *.txt *.md Doxyfile .gitignore .gitmodules .ci* dist/*.desktop \
+                 dist/*.svg dist/*.xml; then
+    echo Trailing whitespace found, aborting
+    exit 1
+fi
+
+# Default clang-format points to default 3.5 version one
+CLANG_FORMAT=clang-format-15
+$CLANG_FORMAT --version
+
+if [ "$GITHUB_EVENT_NAME" = "pull_request" ]; then
+    # Get list of every file modified in this pull request
+    files_to_lint="$(git diff --name-only --diff-filter=ACMRTUXB $COMMIT_RANGE | grep '^src/[^.]*[.]\(cpp\|h\)$' || true)"
+else
+    # Check everything for branch pushes
+    files_to_lint="$(find src/ -name '*.cpp' -or -name '*.h')"
+fi
+
+# Turn off tracing for this because it's too verbose
+set +x
+
+for f in $files_to_lint; do
+    d=$(diff -u "$f" <($CLANG_FORMAT "$f") || true)
+    if ! [ -z "$d" ]; then
+        echo "!!! $f not compliant to coding style, here is the fix:"
+        echo "$d"
+        fail=1
+    fi
+done
+
+set -x
+
+if [ "$fail" = 1 ]; then
+    exit 1
+fi