fdo#64232: fix theme fonts application

We were applying the theme fonts taking only into account the
<a:latin>, <a:ea> and <a:cs> tags to set latin, eastAsia and cs fonts
respectively. Word uses in addition the <a:font> tags to define a
font name per language, in combination with <w:themeFontLang> tag in
settings.xml file which indicates the default language for latin, cs
and eastAsia. For example, with <w:themeFontLang w:bidi="he-IL" />
the default language for cs text is Hebrew, if the theme says
<a:font script="Hebr" typeface="Arial"/> then the selected font for
those sections of text should be Arial.

In theory, Word would also be able to define the language for a
specific section of text and that should receive the proper font from
the theme definition, but we are currently not supporting it. More
info: http://blogs.msdn.com/b/officeinteroperability/archive/2013/04/
22/office-open-xml-themes-schemes-and-fonts.aspx

Main changes in this patch:
* Added setter and getter values to ThemeTable and SettingsTable to
  be able to pass the values read in <w:themeFontLang> tag.
* Added support to manage the values in <w:themeFontLang> and
  <a:font> tags, which used to be discarded.
* Added methods to translate locale definitions to the language codes
  used in the <a:font>, using a table defined in the link above.

Change-Id: I78e6da5feab34cb9ff770a2c60e2f7229e7fb140
fdo#64232: export w:themeFontLang setting to docx

We store the values of themeFontLang tag to the document grab bag so
we can save it back to the document on export time.

Added unit tests to check that the attribute is properly set back and
also that the theme fonts are correctly applied to the text.

Change-Id: Ia54c513796ba38a571396ca7b72dfd28463c15fd