widget: Add proper accessors for the 'opacity' property

To avoid collision with the property system and comply with the
API guidelines.

lib/wibox/hierarchy.lua

@@ -272,7 +272,7 @@ function hierarchy:draw(context, cr)
 
     -- Draw if needed
     if not empty_clip(cr) then
-        local opacity = widget.opacity
+        local opacity = widget:get_opacity()
         local function call(func, extra_arg1, extra_arg2)
             if not func then return end
             if not extra_arg2 then

lib/wibox/widget/base.lua

@@ -50,12 +50,19 @@ end
 -- transparent while 1 is fully opaque.
 -- @function set_opacity
 function base.widget:set_opacity(o)
-    if o ~= self.opacity then
-        self.opacity = o
+    if o ~= self._private.opacity then
+        self._private.opacity = o
         self:emit_signal("widget::redraw")
     end
 end
 
+--- Get the widget opacity.
+-- @treturn number The opacity (between 0 and 1)
+-- @function get_opacity
+function base.widget:get_opacity()
+    return self._private.opacity
+end
+
 --- Set the widget's width
 -- @tparam number|nil s The width that the widget has. `nil` means to apply the
 -- default mechanism of calling the `:fit` method. A number overrides the result
@@ -535,7 +542,7 @@ function base.make_widget(proxy, widget_name, args)
     ret.visible = true
 
     -- Widget is fully opaque
-    ret.opacity = 1
+    ret._private.opacity = 1
 
     -- Differentiate tables from widgets
     rawset(ret, "is_widget", true)