GTK3: Switch to using CSS syntax for specifying fonts from Pango. Apparently the theme warnings produced on GTK3 were from using Pango syntax for specifying fonts, instead of CSS syntax. Can't find any information about when CSS syntax was introduced... So hopefully it works through the entire GTK3 series. Also fix some GCC warnings in the new dwtest code.

# Set these here to change the version for all platforms
DW_MAJOR_VERSION=3
DW_MINOR_VERSION=2