Activate GitLab CI and GitLab Pages (HTML docs).

The GitLab CI control file enables continuous builds after every commit
and generates the (doxygen) HTML documentation.

The documentation is put online on GitLab pages at:

https://fltk.gitlab.io/fltk/


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.4@12964 ea41ed52-d2ee-0310-a9c1-e6b18d33e121