allow groupbox to be center-aligned via center_aligned parameter

.gitignore

@@ -9,6 +9,7 @@ MANIFEST
 /dist
 # Files generated by setup.py
 qtile.egg-info/
+.eggs
 # Vim swap files
 *.swp
 *.swo

libqtile/widget/groupbox.py

@@ -37,6 +37,7 @@
 class _GroupBase(base._TextBox, base.PaddingMixin, base.MarginMixin):
     defaults = [
         ("borderwidth", 3, "Current group border width"),
+        ("center_aligned", False, "center-aligned group box"),
     ]
 
     def __init__(self, **config):
@@ -96,10 +97,16 @@ def drawbox(self, offset, text, bordercolor, textcolor, rounded=False,
             self.padding_x,
             self.padding_y
         )
+        y = self.margin_y
+        if self.center_aligned:
+            for t in base.MarginMixin.defaults:
+                if t[0] == 'margin':
+                    y += (self.bar.height - framed.height) / 2 - t[1]
+                    break
         if block:
-            framed.draw_fill(offset, self.margin_y, rounded)
+            framed.draw_fill(offset, y, rounded)
         else:
-            framed.draw(offset, self.margin_y, rounded)
+            framed.draw(offset, y, rounded)
 
 
 class AGroupBox(_GroupBase):