Attempt to get rid of the remaining Ubuntu substitution mess

"Today" is 0.4.15-dev-5328-g7e9f1e6

Ubuntu is still referenced in some of our Themes (and as a consequence also in some "theme-fix-up-substitution-hacks"), although the font is no longer present since
0.4.14-dev-20-g2f4fb90 for master
and for older releases since:
0.4.13-release-103-g8bfda40
0.4.12-release-133-gd32e335
0.4.11-release-144-g75fc4f1
0.4.10-release-159-g6543bf3
0.4.9-release-171-g68befdd
0.4.8-release-175-g5b98a1e
0.4.7-release-199-ga4f921f

That is no regression, because the font Ubuntu does also not exist on MS Windows.

This isn't a big thing actually, because the themes do look quite decent even without that, falling back to the Shell font, especially since we have added better glyph support in Tahoma over time. Better candidates are Tahoma and/or Comic Sans.
Still I want to clean up that mess during 0.4.15-dev'ing. I will propose a PR for that soon(TM).