Adds a PanelMixin plugin mixin class

Intended to allow rendering of custom panels on pages

InvenTree/plugin/samples/integration/custom_panel_sample.py

@@ -0,0 +1,22 @@
+"""
+Sample plugin which renders custom panels on certain pages
+"""
+
+from plugin import IntegrationPluginBase
+from plugin.mixins import PanelMixin
+
+
+class CustomPanelSample(PanelMixin, IntegrationPluginBase):
+    """
+    A sample plugin which renders some custom panels.
+    """
+
+    PLUGIN_NAME = "CustomPanelExample"
+    PLUGIN_SLUG = "panel"
+    PLUGIN_TITLE = "Custom Panel Example"
+
+    def get_custom_panels(self, page, instance, request):
+
+        print("get_custom_panels:")
+
+        return []