use Gsettings 'pluma_prefs_manager' instead

When using intltool, the taglist creates a big xml file containing the
local translation string, so that it has to be compressed.

It looks like this:
```
<TagGroup sort="true" name="XHTML 1.0 - Tags">
  <Tag name="Abbreviated form">
    ...
  </Tag>
  <Tag name="...">
    ...
  </Tag>
  ...
</TagGroup>
<TagGroup xml:lang="af" sort="true" name="XHTML 1.0 - Tags">
  <Tag name="Afgekorte vorm" xml:lang="af">
    ...
  </Tag>
  <Tag name="..." xml:lang="af">
    ...
  </Tag>
  ...
</TagGroup>
<TagGroup xml:lang="..." sort="true" name="XHTML 1.0 - Tags">
</TagGroup>
```

Obviously, it wastes space and download bandwidth.

When switch from intltool to gettext, it does not generate a similar
huge xml file. It only get the translate string into pot file from the
xml file, and the original xml file has not changed.

This patch let taglist-plugin read the local translation string directly
from the mo file, so that it can work as before.

TODO: we need to improve and optimize the taglist plugin code and
drop some extra code.

When xml file is generated using intltool, the resulting xml file deletes the
comment line, but when using gettext, it keeps the comment line.