color: move '_colormode' to the core 'ui' class

Having all 'ui' objects aware of 'color' allows us to update the core code to
handle color. The mode will stay 'None' in the default case so that will not
introduce any changes.

hgext/color.py

@@ -297,7 +297,6 b' def _modesetup(ui, coloropt):'
     return None
 
 class colorui(uimod.ui):
-    _colormode = None
     def write(self, *args, **opts):
         if self._colormode is None:
             return super(colorui, self).write(*args, **opts)

mercurial/ui.py

@@ -127,6 +127,8 b' def _catchterm(*args):'
     raise error.SignalInterrupt
 
 class ui(object):
+    # color mode: see mercurial/color.py for possible value
+    _colormode = None
     def __init__(self, src=None):
         """Create a fresh new ui object if no src given