Remove unused font specifications in widgets

These popups are styled just fine, no need to specify the default font
again. Same for widgets whose name starts with "headline", these
automatically apply the headline font.
Rename font shapes

None of those numbers actually corresponded to the font's size, so they
were misleading at best. Now named after most prominent use case --
usually there only is one such use case anyways.

14_black => default
14_bold => default_bold
14_bold_light => default_load
16_black_unifont => unifont
18 => cityinfo
small_black => resourcebar
small_tooltip => pickbelt
Remove unused font definitions
Reorder font definitions

By size. This now mixes in the unifont definition for Libertine.
Remove tooltip style

Turns out it is identical to the default style.
Adapt unifont fontdef to recent changes
Merge pull request #2003 from unknown-horizons/font

Fontdef cleanup -- the intermediate commits might not be stable with
languages that do not overwrite Unifont with Libertine. Sorry about that.