gtk: Fix warnings for some uses of GtkLinkButton
gtk: Fix warnings for some uses of GtkLinkButton

Not setting a URI but catching the activate-link signal is a
valid use of GtkLinkButton, but we shouldn't allow showing a
popup menu which offers to copy the URI if there's none.

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