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.