Add solid and modal attributes.

The `solid` attribute determines whether to propagate events downwards.
When the widget being interacted with is not solid, events propagate down
to the next layout or to underlying handlers (for example love callbacks).
This is set by the theme and works mostly behind the scenes.

The `modal` attribute can be placed on a layout root to prevent input events
from propagating downwards, even those that fall outside of the layout.

luigi/widget/slider.lua

@@ -73,16 +73,14 @@ return function (self)
     self:onReshape(function (event)
         local x1, y1, w, h = self:getRectangle(true, true)
         local x2, y2 = x1 + w, y1 + h
+        local dim = math.min(w, h)
+        thumb.width, thumb.height = dim, dim
         if self.flow == 'x' then
-            thumb.width = 0
-            thumb.height = false
             local halfThumb = thumb:getWidth() / 2
             x1, x2 = x1 + halfThumb, x2 - halfThumb
             spacer.width = self.value * (x2 - x1)
             spacer.height = false
         else
-            thumb.width = false
-            thumb.height = 0
             local halfThumb = thumb:getHeight() / 2
             y1, y2 = y1 + halfThumb, y2 - halfThumb
             spacer.width = false