Merge branch 'gtk' into 'master'

Allow graph_tool.draw.draw_hierarchy to be used without GTK installed

See merge request !29
4 jobs for master in 236 minutes and 33 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed job_clang_amd64 #4716
amd64

03:56:33

passed job_gcc_amd64 #4715
amd64

00:53:34

failed job_clang_amd64 #4712
amd64

00:00:51

failed job_gcc_amd64 #4711
amd64

00:00:51