Introduce "updates" data source
Remove leftover stuff regarding setting text colours
Small simplification of data source creation
Add sysname data source
Add kernel, machine, hostname_short data sources
Add conky_{version,build_date,build_arch} data sources
Move assorted classes/functions to util.hh
Move lua_traits classes to a separate file, add point traits
Fix a bug in strprintf
Improve "syntax error" error message
Add a "fixed" layout engine

also, factor out common layout engine code into layout_engine class.