Fix a weird length calculation issue on textboxes

I thought it was smart to only assign the text to the layout when it's
drawn (see https://github.com/qtile/qtile/pull/1367/files#r300469727),
but it seems that it's used for the calculation of the widget's length
beforehand and result in buggy behaviors that way (either there is a delay
before the widget is drawn at the proper size, or it is just drawn with a wrong
length and stays that way). So let's revert to a property setter method; plus
it will give a better output for info().

libqtile/widget/base.py

@@ -299,9 +299,19 @@ class _TextBox(_Widget):
     def __init__(self, text=" ", width=bar.CALCULATED, **config):
         self.layout = None
         _Widget.__init__(self, width, **config)
-        self.text = text
+        self._text = text
         self.add_defaults(_TextBox.defaults)
 
+    @property
+    def text(self):
+        return self.fmt.format(self._text)
+
+    @text.setter
+    def text(self, value):
+        self._text = value
+        if self.layout:
+            self.layout.text = self.text
+
     @property
     def foreground(self):
         return self._foreground
@@ -353,7 +363,7 @@ def _configure(self, qtile, bar):
         )
 
     def calculate_length(self):
-        if self.text:
+        if self._text:
             return min(
                 self.layout.width,
                 self.bar.width
@@ -366,7 +376,6 @@ def draw(self):
         if self.offsetx is None:
             return
         self.drawer.clear(self.background or self.bar.background)
-        self.layout.text = self.fmt.format(self.text)
         self.layout.draw(
             self.actual_padding or 0,
             int(self.bar.height / 2.0 - self.layout.height / 2.0) + 1