unify color specification

colors were a mixture of 4bpp integers (0=white, 15=black) and
fractional blackness levels (0=white, 1.0=black) before. This is
now unified to use the color specification of the Blitbuffer API.

frontend/ui/widget/textboxwidget.lua

@@ -13,7 +13,7 @@ local TextBoxWidget = Widget:new{
     text = nil,
     face = nil,
     bold = nil,
-    fgcolor = 1.0, -- [0.0, 1.0]
+    fgcolor = Blitbuffer.COLOR_BLACK,
     width = 400, -- in pixels
     height = nil,
     first_line = 1,