Add CuraView, which does something similar to CuraStage

So instead of relying on strange activeViewProxy, it's up to the CuraView to provide a set of components.
These can subsequently be used by the active stage again.

CURA-5829

cura/CuraView.py

@@ -0,0 +1,24 @@
+# Copyright (c) 2018 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from PyQt5.QtCore import pyqtProperty, QUrl
+
+from UM.View.View import View
+
+
+# Since Cura has a few pre-defined "space claims" for the locations of certain components, we've provided some structure
+# to indicate this.
+#   MainComponent works in the same way the MainComponent of a stage.
+#   the stageMenuComponent returns an item that should be used somehwere in the stage menu. It's up to the active stage
+#   to actually do something with this.
+class CuraView(View):
+    def __init__(self, parent = None) -> None:
+        super().__init__(parent)
+
+    @pyqtProperty(QUrl, constant = True)
+    def mainComponent(self) -> QUrl:
+        return self.getDisplayComponent("main")
+
+    @pyqtProperty(QUrl, constant = True)
+    def stageMenuComponent(self) -> QUrl:
+        return self.getDisplayComponent("menu")