set use preedit

[Botond Botyanszki <gtk-i18n siliconium net>]

Hello,

The GtkIMContext interface provides an API gtk_im_context_set_use_preedit
that the application can call to notify the input method whether it
supports preedit or not.
I think it is usually called by the application after GtkIMMultiContext is
instantiated. I have only seen gnome-terminal actually using this API. 

The problem is that if the input method is changed (by switching to a
different one in the MultiContext menu), the newly loaded module cannot
know about the preedit support, because the set_use_preedit function isn't
called again.

e.g. if I start gnome-terminal with 
`GTK_IM_MODULE="my-module" gnome-terminal`
then my-module gets the use_preedit value, but if I start gnome-terminal
without any im modules preloaded and then switch to my-module from the
menu, it won't get the set_use_preedit flag. 

Is this a faulty design in GtkIMContext, or should the application
(gnome-terminal/libvte) do something else in addition ?