allow image widgets to do alpha-blitting

when the image (e.g. a PNG) does contain an alpha channel, that can
be honored by ImageWidget. It doesn't do so by default for compatibility
(arguably, we should change that in the future), it has to be enabled
by setting the "alpha" property to "true" (boolean, not string).

frontend/ui/widget/imagewidget.lua

@@ -37,6 +37,8 @@ local ImageWidget = Widget:new{
     height = nil,
     -- if autoscale is true image will be rescaled according to screen dpi
     autoscale = false,
+    -- when alpha is set to true, alpha values from the image will be honored
+    alpha = false,
     _bb = nil
 }
 
@@ -108,7 +110,11 @@ function ImageWidget:paintTo(bb, x, y)
         w = size.w,
         h = size.h
     }
-    bb:blitFrom(self._bb, x, y, 0, 0, size.w, size.h)
+    if self.alpha == true then
+        bb:alphablitFrom(self._bb, x, y, 0, 0, size.w, size.h)
+    else
+        bb:blitFrom(self._bb, x, y, 0, 0, size.w, size.h)
+    end
     if self.invert then
         bb:invertRect(x, y, size.w, size.h)
     end