Add merge request template with check list for reviewers

.gitlab/merge_request_templates/merge_request.md

+**Add your decription here**
+
+
+# Reviewer Checklist
+
+## General code quality
+- [ ] naming conventions are met (see .clang-tidy for detailed information)
+- [ ] no static analyzer warnings in new code parts (e.g., use clang-tidy for checking)
+
+## General usability
+- [ ] old versions of pet-files are still loadable
+
+## Only if changes in UI
+- [ ] new elements are also saved and loaded from pet-file
+- [ ] check if tab order is still correct
+- [ ] all new SpinBoxes are promoted
+- [ ] new keybindings added to `Petrack::keyBindings()`