Add extruders model

Hasn't been tested much because there is no display for this in the front-end yet. It needs a setting data type to go with in order to display it. This model is for the drop-down boxes to select an extruder to print, for instance, the support with.

Contributes to issues CURA-1278 and CURA-351.

cura/ExtrudersModel.py

@@ -0,0 +1,55 @@
+# Copyright (c) 2016 Ultimaker B.V.
+# Cura is released under the terms of the AGPLv3 or higher.
+
+from PyQt5.QtCore import Qt
+
+import cura.ExtruderManager
+import UM.Qt.ListModel
+
+##  Model that holds extruders.
+#
+#   This model is designed for use by any list of extruders, but specifically
+#   intended for drop-down lists of extruders in place of settings.
+class ExtrudersModel(UM.Qt.ListModel):
+    ##  Human-readable name of the extruder.
+    NameRole = Qt.UserRole + 1
+
+    ##  Colour of the material loaded in the extruder.
+    ColourRole = Qt.UserRole + 2
+
+    ##  Index of the extruder, which is also the value of the setting itself.
+    #
+    #   An index of 0 indicates the first extruder, an index of 1 the second
+    #   one, and so on. This is the value that will be saved in instance
+    #   containers.
+    IndexRole = Qt.UserRole + 3
+
+    ##  Initialises the extruders model, defining the roles and listening for
+    #   changes in the data.
+    #
+    #   \param parent Parent QtObject of this list.
+    def __init__(self, parent = None):
+        super().__init__(parent)
+
+        self.addRoleName(self.NameRole, "name")
+        self.addRoleName(self.ColourRole, "colour")
+        self.addRoleName(self.IndexRole, "index")
+
+        #Listen to changes.
+        manager = cura.ExtruderManager.ExtruderManager.getInstance()
+        manager.extrudersChanged.connect(self._updateExtruders())
+
+    ##  Update the list of extruders.
+    #
+    #   This should be called whenever the list of extruders changes.
+    def _updateExtruders(self):
+        self.clear()
+        manager = cura.ExtruderManager.ExtruderManager.getInstance()
+        for index, extruder in enumerate(manager):
+            item = { #Construct an item with only the relevant information.
+                "name": extruder.name,
+                "colour": extruder.material.getMetaDataEntry("color_code", default = "#FFFF00"),
+                "index": index
+            }
+            self.appendItem(item)
+        self.sort(lambda item: item["index"])