widgets: Fix faulty save_state() under Python 3

The result of `toBase64().data()`, bytes data, was wrapped into a string
using `ustr()` causing the resulting trying to be a *representation* of
the bytes (wrapped into `b''`), thus being buggy data.

I've replaced this with a proper ".decode()" call.

I've tried keeping it as bytes, but that didn't go through
serialization.

Fixes #236

Signed-off-by: Virgil Dupras <hsoft@hardcoded.net>

cola/widgets/standard.py

@@ -10,7 +10,6 @@
 from cola import gitcfg
 from cola import qtcompat
 from cola import settings
-from cola.compat import ustr
 
 
 class WidgetMixin(object):
@@ -101,7 +100,7 @@ def export_state(self):
         state = WidgetMixin.export_state(self)
         windowstate = self.saveState(self.widget_version)
         state['lock_layout'] = self.lock_layout
-        state['windowstate'] = ustr(windowstate.toBase64().data())
+        state['windowstate'] = windowstate.toBase64().data().decode('ascii')
         return state
 
     def apply_state(self, state):