gitlab-ci: add basic CI template for KDE CI

Based on https://invent.kde.org/sysadmin/ci-tooling/wikis/Gitlab-CI

.gitlab-ci.yml

+include:
+  - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-before.yml
+  - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-kdevelop-linux.yml
+  - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-kdevelop-freebsd.yml
+  - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-kdevelop-windows.yml