[core] Add a "scrollable" modifier for widget texts callbacks

If a module defines a callback for a widget's text, an optional
decorator "@bumblebee.output.scrollable" can be used to make the text
scrollable.

In those cases, the desired width is set to (in decreasing order of
priority):
1. whatever the widget defines as "theme.width"
2. whatever the theme defines as "width" for the module
3. whatever the commandline parameter "width" for the module is set to
4. 30 (determined by unfair dice roll)

see #27

bumblebee/output.py

@@ -8,6 +8,27 @@ import uuid
 
 import bumblebee.store
 
+def scrollable(func):
+    def wrapper(module, widget):
+        text = func(module, widget)
+        width = widget.get("theme.width", module.parameter("width", 30))
+        widget.set("theme.minwidth", "A"*width)
+        if len(text) <= width:
+            return text
+        # we need to shorten
+        start = widget.get("scrolling.start", -1)
+        direction = widget.get("scrolling.direction", "right")
+        start += 1 if direction == "right" else -1
+        widget.set("scrolling.start", start)
+        if width + start >= len(text):
+            widget.set("scrolling.direction", "left")
+        if start <= 0:
+            widget.set("scrolling.direction", "right")
+        text = text[start:width+start]
+
+        return text
+    return wrapper
+
 class Widget(bumblebee.store.Store):
     """Represents a single visible block in the status bar"""
     def __init__(self, full_text="", name=""):
@@ -83,13 +104,14 @@ class I3BarOutput(object):
                 "background": self._theme.separator_bg(widget),
                 "separator_block_width": self._theme.separator_block_width(widget),
             })
+        width = self._theme.minwidth(widget)
         self._widgets.append({
             u"full_text": full_text,
             "color": self._theme.fg(widget),
             "background": self._theme.bg(widget),
             "separator_block_width": self._theme.separator_block_width(widget),
             "separator": True if separator is None else False,
-            "min_width": self._theme.minwidth(widget),
+            "min_width": width + "A"*(len(prefix) + len(suffix)) if width else None,
             "align": self._theme.align(widget),
             "instance": widget.id,
             "name": module.id,

bumblebee/theme.py

@@ -139,7 +139,7 @@ class Theme(object):
                 state_themes.append(self._get(widget, state, {}))
 
         value = self._defaults.get(name, default)
-        value = widget.get("theme-{}".format(name), value)
+        value = widget.get("theme.{}".format(name), value)
         value = self._cycle.get(name, value)
         value = module_theme.get(name, value)