cc-language-chooser: Reset dialog to original state when used the 2nd time

The language chooser dialog is used in the users panel repeatedly.
When we click to show more languages and/or select language, close
the dialog and show it again e.g. for different user, the chooser
isn't in an original state. So filter entry isn't hidden (and all
languages are shown) and/or the last selected language is focused.
But the dialog should be in the original state when it is open.

This patch changes cc_language_chooser_clear_filter to clear the
dialog into the original state that we can reuse the dialog later.

https://bugzilla.gnome.org/show_bug.cgi?id=738077