Carry current context content type into fake context when switching

When focus moves out of a context to the fake context we should keep
the content type properties as they were in the focused out context.

This allows panel implementations to show the proper UI while input
focus moves away temporarily from a text entry.

E.g. a password entry is focused and the user clicks on the panel
UI. At this point ibus switches the currently focused context to the
fake context and emits set-content-type with the fake context
properties which are empty and thus the panel reacts to it. This isn't
the intended behavior though since, from a user POV, the password
entry is still the context that will be getting text input when the
panel UI gets dismissed.

R=shawn.p.huang@gmail.com
BUG=https://bugzilla.gnome.org/show_bug.cgi?id=730628

Review URL: https://codereview.appspot.com/176260043
Patch from Rui Matos <tiagomatos@gmail.com>.