Remove a bunch of unused functionalty from agi::Exception
Replace wxString::Format with agi::format

It's modestly faster, significantly more type-safe, and doesn't assert
when there's too few arguments, which causes problems for plural forms.

Closes #1733.
Extract some of the string formatter implementation to format.cpp
Use iconv rather than std::wstring_convert

libstdc++ 4.8 does not have <codecvt>.
Fix compilation with UTF-8 wxString
Disable decoders for dumb rarely-used formats which have huge data tables
Enable ffmpeg's png decoder
Kill ScintillaTextCtrl and just always use the UTF-8 methods

Trivially improves performance and reduces the opportunities for wxSTC
to fuck up.
Update wxWidgets