Advise user to install poppler-data if applicable

[dg1727]

In order to display CJK fonts, Evince needs the package "poppler-data" to be installed. If "poppler-data" isn't installed, any CJK text in a document is invisible, and Evince doesn't indicate to the user why the text is invisible.

Suggested enhancement to Evince:
(modified from https://bugs.launchpad.net/ubuntu/+source/poppler/+bug/197537/comments/26 )
'What needs to be done is for Evince to display a warning like "CJK font support is included in the package poppler-data, which is currently not installed on your system" in these cases.'

An intended solution has been made (documented in https://bugs.launchpad.net/ubuntu/+source/language-selector/+bug/893920 )
by modifying language-selector, starting with Ubuntu 12.04, to install poppler-data unconditionally.

Even so, there are still some reasons for the suggested enhancement to Evince:
 * Someone may un-install poppler-data, either accidentally or on purpose, (if Evince was un-installed as part of this, then Evince may then be re-installed without poppler-data,) then try to view a file in Evince that contains CJK fonts.
 * I don't know much about language-selector, so I suppose there may be cases when language-selector doesn't run, either on upgrade from a version of Ubuntu prior to 12.04, or on a new install of 12.04 or higher.
 * If the issue is addressed upstream (that is, in Evince), then users of other Linux distros would benefit. I am reporting this issue against Ubuntu instead of upstream because of the relevancy of language-selector, which is specific to Ubuntu.

The issue of invisible "CJK" fonts can affect PDF documents that contain only Latin-alphabet characters but were produced by software that is set up for CJK speakers. So the issue can affect non-CJK-speaking Evince users.

My Evince package version is 2.32.0-0ubuntu12.4 on Ubuntu 11.04 (natty).