[THEMES] Eliminates the last references to "Open Sans" font.
This is an addendum to 0.4.13-dev-765-g 04a361d091
"Open Sans" has been replaced by "Trebuchet MS" back then.
[DOC] Mention trebuc*.ttf as being based on 'Open Sans Fonts'
An addendum to 0.4.14-dev-20-g 2f4fb903b4
because since then we don't have the Ubuntu font anymore.
The substitutes are also not needed any longer.
We can use Tahoma here without causing any change in the current rendering,
because for all languages the former
Ubuntu substitutions did point either to Tahoma,
or to the same thing, that Tahoma atm points to
(for those language that do require additional glyphs).
This way we do not only get the substitutions closer to 2k3sp2, but will also
simplify our maintenance and testing, because the same font is guaranteed to be used then
for all themes: Classic, Blackshade and Lautus: The font which has the needed glyphs for
that specific language.
E.g.
"FreeSans" for Hindi,
"Tahoma" for most Western languages, and
"Droid Sans Fallback" for Chinese and Japanese language.
Things are cleaner and simpler this way.
