There are some widgets (e.g. fg/bg color buttons in the ColorBar, and
ContextBar's check-boxes) that use a "mini font". We could setup the mini
font in onInitTheme(), but the whole program is not ready to do something
like that (there are too much child_spacing/borders values that are set
outside onInitTheme).
A better way is to ask to the theme itself (Theme::getWidgetFont())
about what font to use for each specific widget. And the Widget::m_font
field can act as a cache of this requested font. So now the "mini font"
is specified in a SkinProperty's flag.
