[*] Rework theme customization.

This commit allows more flexible and consistent theme overriding
rules for each Widget. There are 3 levels of customization.

1. Global theme (yui.theme) this is the default theme for every Ui
2. Ui theme, this overrides the global theme and provides a default
   for every Widget of the Ui
3. Widget theme, this overrides the Ui theme for a single widget

This commit also allows themes to specify a font field.
This replaces the sparse color, font and cornerRadius field often
provided by each Widget.
Widgets and Uis may also partially override a theme by specifying
only a few fields.

slider.lua

@@ -24,7 +24,6 @@ Slider.__index = Slider
 -- @field vertical (boolean) true for vertical slider, false or nil for horizontal slider
 -- @field value (number) default value
 -- @field step (number) number of slider's steps
--- @field cornerRadius (number) radius for rounded corners
 -- @table SliderAttributes
 
 
@@ -33,8 +32,6 @@ Slider.__index = Slider
 function Slider:new(args)
     self = setmetatable(args, self)
 
-    self.color = self.color or core.theme.color
-    self.cornerRadius = self.cornerRadius or core.theme.cornerRadius
     self.vertical = self.vertical or false
     self.min = self.min or 0
     self.max = self.max or 1
@@ -87,7 +84,8 @@ end
 function Slider:draw()
     local x,y,w,h = self.x,self.y,self.w,self.h
     local r = math.min(w,h) / 2.1
-    local c = self:colorForState()
+    local color, _, cornerRadius = core.themeForWidget(self)
+    local c = core.colorForWidgetState(self, color)
     local fraction = (self.value - self.min) / (self.max - self.min)
 
     local xb, yb, wb, hb  -- size of the progress bar
@@ -99,8 +97,8 @@ function Slider:draw()
         xb, yb, wb, hb = x,y, w*fraction, h
     end
 
-    core.drawBox(x,y,w,h, c, self.cornerRadius)
-    core.drawBox(xb,yb,wb,hb, {bg=c.fg}, self.cornerRadius)
+    core.drawBox(x,y,w,h, c, cornerRadius)
+    core.drawBox(xb,yb,wb,hb, { bg = c.fg }, cornerRadius)
 
     if self:isFocused() then
         love.graphics.setColor(c.fg)