Button: fix enableDisable when multilines

Also add support for a enabled_func() property, and
ensure its enable/disable state on each repaint.

frontend/ui/widget/button.lua

@@ -252,6 +252,9 @@ function Button:enable()
     if not self.enabled then
         if self.text then
             self.label_widget.fgcolor = Blitbuffer.COLOR_BLACK
+            if self.label_widget.update then -- using a TextBoxWidget
+                self.label_widget:update() -- needed to redraw with the new color
+            end
         else
             self.label_widget.dim = false
         end
@@ -263,6 +266,9 @@ function Button:disable()
     if self.enabled then
         if self.text then
             self.label_widget.fgcolor = Blitbuffer.COLOR_DARK_GRAY
+            if self.label_widget.update then
+                self.label_widget:update()
+            end
         else
             self.label_widget.dim = true
         end
@@ -289,6 +295,14 @@ function Button:enableDisable(enable)
     end
 end
 
+function Button:paintTo(bb, x, y)
+    if self.enabled_func then
+        -- state may change because of outside factors, so check it on each painting
+        self:enableDisable(self.enabled_func())
+    end
+    InputContainer.paintTo(self, bb, x, y)
+end
+
 function Button:hide()
     if self.icon and not self.hidden then
         self.frame.orig_background = self.frame.background

frontend/ui/widget/buttontable.lua

@@ -53,6 +53,7 @@ function ButtonTable:init()
                 icon_height = btn_entry.icon_height,
                 align = btn_entry.align,
                 enabled = btn_entry.enabled,
+                enabled_func = btn_entry.enabled_func,
                 callback = function()
                     if self.show_parent and self.show_parent.movable then
                         self.show_parent.movable:resetEventState()