Bug 695895: Fix gtk3-demo on Windows
Bug 695895: Fix gtk3-demo on Windows

In commit 4e41577b, we are using g_content_type_is_a() to determine how
to display the demo resources in the right pane of the gtk3-demo program.

Use g_content_type_get_mime_type(), so that we can obtain the mime
type of the demo resources on all platforms, as g_content_type_guess()
returns a platform-specific string, as
https://developer.gnome.org/gio/2.35/gio-GContentType.html states.

As .ui files and .css files are normally registered with a different mime
type string on Windows, check for those strings as well.

This will ensure the demo resources can be properly displayed on Windows
as well.

jigpu/gtk,ebassi/gtk,davidgumberg/gtk,grubersjoe/adwaita,ahodesuka/gtk,msteinert/gtk,ebassi/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,chergert/gtk,Lyude/gtk-,Adamovskiy/gtk,jadahl/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,grubersjoe/adwaita,davidgumberg/gtk,jessevdk/gtk,jadahl/gtk,jessevdk/gtk,ahodesuka/gtk,jadahl/gtk,jessevdk/gtk,Lyude/gtk-,Distrotech/gtk2,alexlarsson/gtk,ebassi/gtk,alexlarsson/gtk,alexlarsson/gtk,Adamovskiy/gtk,chergert/gtk,ebassi/gtk,chergert/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,jigpu/gtk,bratsche/gtk-,jessevdk/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,chergert/gtk,Lyude/gtk-,bratsche/gtk-,davidgumberg/gtk,grubersjoe/adwaita,msteinert/gtk,Adamovskiy/gtk,grubersjoe/adwaita,ebassi/gtk,bratsche/gtk-,ahodesuka/gtk,msteinert/gtk,ahodesuka/gtk,grubersjoe/adwaita,jessevdk/gtk,jadahl/gtk,alexlarsson/gtk,Distrotech/gtk2,alexlarsson/gtk,bratsche/gtk-,ebassi/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,msteinert/gtk,grubersjoe/adwaita,bratsche/gtk-,grubersjoe/adwaita,Lyude/gtk-,Adamovskiy/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Distrotech/gtk2,jessevdk/gtk,jigpu/gtk,Lyude/gtk-,msteinert/gtk,chergert/gtk,jigpu/gtk,bratsche/gtk-,ahodesuka/gtk,jigpu/gtk,ahodesuka/gtk,jigpu/gtk,jigpu/gtk,Distrotech/gtk2,Lyude/gtk-,Adamovskiy/gtk,alexlarsson/gtk,Distrotech/gtk2,Distrotech/gtk2,chergert/gtk,Lyude/gtk-,jessevdk/gtk,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,davidgumberg/gtk,ahodesuka/gtk,chergert/gtk