Refactor file loading to detect external modifications and add reload capability

- Extract _load_data() method from load_path() for reuse in reload()
- Add is_extters externally_modified() to track file state changes (mtime/size)
- Replace load_bytes() with load_contents() for better error handling
- Add reload() method to re-read file from disk
- Fix file_state_watcher by properly detecting external changes

plugins/code/event-watchers/prettify_json/prettify_json.py

@@ -13,8 +13,6 @@ from gi.repository import Gtk
 
 
 def add_prettify_json(buffer, menu):
-    menu.append(separator)
-
     def on_prettify_json(menuitem, buffer):
         start_itr, \
         end_itr = buffer.get_start_iter(), buffer.get_end_iter()
@@ -28,4 +26,4 @@ def add_prettify_json(buffer, menu):
 
     item = Gtk.MenuItem(label = "Prettify JSON")
     item.connect("activate", on_prettify_json, buffer)
-    menu.append(item)
+    menu.append(item)