Moved plugins and refactor command system

- Moved plugins to apropriate sub folders
- Refactor command system with new add_command method and rename GetCommandSystemEvent to GetNewCommandSystemEvent
- Add RegisterCommandEvent for dynamic command registration
- Change footer container orientation to VERTICAL
- Add search-highlight tag to source buffer
- Add file change detection (deleted, externally modified) in source_file
- Add JSON prettify option to source view popup menu
- Enable hexpand on VTE widget
- Update plugins_controller_mixin to use widget_registry

plugins/code/completers/example_completer/manifest.json

@@ -0,0 +1,7 @@
+{
+    "name": "Example Completer",
+    "author": "John Doe",
+    "version": "0.0.1",
+    "support": "",
+    "requests": {}
+}