Add a template for bug reports

GitLab issues can use templates to guide users when filing new bug
reports; we should take advantage of this feature.

.gitlab/issue_templates/Bug.md

@@ -0,0 +1,39 @@
+## Steps to reproduce
+
+ 1. ...
+ 2. ...
+ 3. ...
+
+<!--
+  You should try and reproduce with the demos applications available
+  under the `demos` directory, or the test programs in the `tests` directory.
+  Alternatively, please attach a *small and self-contained* example that
+  exhibits the issue.
+-->
+
+## Current behavior
+<!--
+  Please describe the current behaviour
+-->
+
+## Expected outcome
+<!--
+  Please describe the expected outcome
+-->
+
+# Version information
+<!--
+ - which version of GTK+ you are using
+ - what operating system and version
+  - for Linux, which distribution
+ - if you built GTK+ yourself, the list of options used to configure the build
+-->
+
+# Additional information
+<!--
+ - If the bug was a crash, the exact text that was printed out
+   when the crash occurred.
+ - If the bug was a crash, attaching a stack trace obtained using
+   GDB is appreciated; follow the instructions on the wiki:
+   https://wiki.gnome.org/Community/GettingInTouch/Bugzilla/GettingTraces
+-->