Refactored the "connection_type" metadata entry so it can support multiple types.

After a lot of discussion and finding out what the hell was going on,
we figured out we made a pretty big derp by only setting a single connection_type
in the metadata of the machine. What it's actually doing is describing what connection types
have been configured (and not just randomly displaying whatever output device set the value last)

cura/Settings/GlobalStack.py

@@ -42,7 +42,12 @@ class GlobalStack(CuraContainerStack):
         # Per thread we have our own resolving_settings, or strange things sometimes occur.
         self._resolving_settings = defaultdict(set) #type: Dict[str, Set[str]] # keys are thread names
 
+        # Since the metadatachanged is defined in container stack, we can't use it here as a notifier for pyqt
+        # properties. So we need to tie them together like this.
+        self.metaDataChanged.connect(self.configuredConnectionTypesChanged)
+
     extrudersChanged = pyqtSignal()
+    configuredConnectionTypesChanged = pyqtSignal()
 
     ##  Get the list of extruders of this stack.
     #
@@ -63,6 +68,27 @@ class GlobalStack(CuraContainerStack):
     def getLoadingPriority(cls) -> int:
         return 2
 
+    @pyqtProperty("QVariantList", notify=configuredConnectionTypesChanged)
+    def configuredConnectionTypes(self):
+        # Requesting it from the metadata actually gets them as strings (as that's what you get from serializing).
+        # But we do want them returned as a list of ints (so the rest of the code can directly compare)
+        connection_types = self.getMetaDataEntry("connection_type", "").split(",")
+        return [int(connection_type) for connection_type in connection_types if connection_type != ""]
+
+    def addConfiguredConnectionType(self, connection_type):
+        configured_connection_types  = self.configuredConnectionTypes
+        if connection_type not in configured_connection_types:
+            # Store the values as a string.
+            configured_connection_types.append(str(connection_type))
+            self.setMetaDataEntry("connection_type", ",".join(configured_connection_types))
+
+    def removeConfiguredConnectionType(self, connection_type):
+        configured_connection_types = self.configuredConnectionTypes
+        if connection_type in self.configured_connection_types:
+            # Store the values as a string.
+            configured_connection_types.remove(str(connection_type))
+            self.setMetaDataEntry("connection_type", ",".join(configured_connection_types))
+
     @classmethod
     def getConfigurationTypeFromSerialized(cls, serialized: str) -> Optional[str]:
         configuration_type = super().getConfigurationTypeFromSerialized(serialized)