Made default extrusion length a setting

Added "extrusion_default" to settings and made it editable via settings
file. Setting is not written back when the user changes it through the
gui.

printrun/gui.py

@@ -170,7 +170,7 @@ def add_extra_controls(self, root, parentpanel, extra_buttons = None):
 
     esettingspanel = root.newPanel(parentpanel)
     esettingssizer = wx.BoxSizer(wx.HORIZONTAL)
-    root.edist = wx.SpinCtrl(esettingspanel, -1, "5", min = 0, max = 1000, size = (70, -1))
+    root.edist = wx.SpinCtrl(esettingspanel, -1, str(root.settings.extrusion_default), min = 0, max = 1000, size = (70, -1))
     root.edist.SetBackgroundColour((225, 200, 200))
     root.edist.SetForegroundColour("black")
     esettingssizer.Add(root.edist, flag = wx.ALIGN_CENTER | wx.RIGHT, border = 5)

pronsole.py

@@ -244,6 +244,7 @@ class Settings(object):
         self._add(SpinSetting("xy_feedrate", 3000, 0, 50000, _("X && Y manual feedrate"), _("Feedrate for Control Panel Moves in X and Y (mm/min)"), "Printer"))
         self._add(SpinSetting("z_feedrate", 200, 0, 50000, _("Z manual feedrate"), _("Feedrate for Control Panel Moves in Z (mm/min)"), "Printer"))
         self._add(SpinSetting("e_feedrate", 100, 0, 1000, _("E manual feedrate"), _("Feedrate for Control Panel Moves in Extrusions (mm/min)"), "Printer"))
+        self._add(SpinSetting("extrusion_default", 5, 0, 1000, _("Extrusion length"), _("Extrusion/REtraction (mm)"), "Printer"))
         self._add(StringSetting("slicecommand", "python skeinforge/skeinforge_application/skeinforge_utilities/skeinforge_craft.py $s", _("Slice command"), _("Slice command"), "External"))
         self._add(StringSetting("sliceoptscommand", "python skeinforge/skeinforge_application/skeinforge.py", _("Slicer options command"), _("Slice settings command"), "External"))
         self._add(StringSetting("final_command", "", _("Final command"), _("Executable to run when the print is finished"), "External"))