Remove tabs UI from code editor and move to plugin. Enhance plugin system.

- Remove tabs controller, tab widget, and tabs widget files and move to plugin
- Delete plugins/README.txt
- Add register_controller method to controller system for plugin use
- Add error handling for plugin crashes via futures callback

plugins/code/tabs_bar/manifest.json

@@ -0,0 +1,7 @@
+{
+    "name": "Tabs Bar",
+    "author": "ITDominator",
+    "version": "0.0.1",
+    "support": "",
+    "requests": {}
+}