GtkIMModule: Use default screen when determining context id
GtkIMModule: Use default screen when determining context id

This code is called early on, without a window, and then later on
with a window. Currently, it returns different results for these
cases when the setting contains a value. That leads to pointless
construction and destruction of im contexts. Instead, just look
at the settings of the default screen. In practice, there is only
one screen, ever.

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