Add debounced text change handling and modified file indicator for tabs

- Debounce word completer refresh with 1500ms timeout to reduce overhead
- Add visual indicator (file-changed class) for modified files in tabs
- Refactor buffer switching into signal_mapper for DRY code
- Fix handler ID indices after adding _after_changed signal
- Move set_modified(False) after successful file write in save()

plugins/code/markdown_preview/plugin.py

@@ -42,7 +42,7 @@ class Plugin(PluginCode):
     def run(self):
         ...
 
- 
+
 class Handler:
     @staticmethod
     def execute(