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/textwidget.lua

@@ -2,6 +2,7 @@ local Widget = require("ui/widget/widget")
 local Screen = require("ui/screen")
 local RenderText = require("ui/rendertext")
 local Geom = require("ui/geometry")
+local Blitbuffer = require("ffi/blitbuffer")
 
 --[[
 A TextWidget puts a string on a single line
@@ -10,7 +11,7 @@ local TextWidget = Widget:new{
     text = nil,
     face = nil,
     bold = nil,
-    fgcolor = 1.0, -- [0.0, 1.0]
+    fgcolor = Blitbuffer.COLOR_BLACK,
     _bb = nil,
     _length = 0,
     _height = 0,