build: Do not call gtk_init()
build: Do not call gtk_init()

It needs to open a display connection, which is obviously going to fail
miserably on any headless build machine.

Instead, we need to find where we started requiring to initialize GTK
when calling a get_type() function, and stop doing that.

This commit and commit 15cc85db295b3d54f85c541b3c82d7b703c851c8 fully
revert commit 6838861d26bfec9af53438022bd1f5b60bab3bf4.

alexlarsson/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,chergert/gtk,jadahl/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,chergert/gtk,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,jigpu/gtk