feat: Complete plugin lifecycle management with lazy loading and runtime reload

Major changes:
- Add unload() method to all plugins for proper cleanup (unregister commands/providers/LSP clients, destroy widgets, clear state)
- Implement lazy widget loading via "show" signal across all containers
- Add autoload: false manifest option for manual/conditional plugin loading
- Add Plugins UI with runtime load/unload toggle via Ctrl+Shift+p
- Implement controller unregistration system with proper signal disconnection
- Add new events: UnregisterCommandEvent, GetFilesEvent, GetSourceViewsEvent, TogglePluginsUiEvent
- Fix signal leaks by tracking and disconnecting handlers in widgets (search/replace, LSP manager, tabs, telescope, markdown preview)
- Add Save/Save As to tabs context menu
- Improve search/replace behavior (selection handling, focus management)
- Add telescope file initialization from existing loaded files
- Refactor plugin reload watcher to dynamically add/remove plugins on filesystem changes
- Add new plugins: file_history, extend_source_view_menu, godot_lsp_client
- Fix bug in prettify_json (undefined variable reference)

plugins/code/ui/markdown_preview/plugin.py

@@ -51,6 +51,18 @@ class Plugin(PluginCode):
 
         self.emit_to("source_views", event)
 
+    def unload(self):
+        event = Event_Factory.create_event("unregister_command",
+            command_name = "tggle_markdown_preview",
+            command      = Handler,
+            binding_mode = "released",
+            binding      = "<Shift><Control>m"
+        )
+
+        self.emit_to("source_views", event)
+
+        markdown_preview.destroy()
+
     def run(self):
         ...
 
@@ -67,4 +79,14 @@ class Handler:
         if not markdown_preview.can_hide:
             markdown_preview.can_hide = True
 
-        markdown_preview.popdown() if markdown_preview.is_visible() else markdown_preview.popup()
+        if markdown_preview.is_visible():
+            markdown_preview.popdown()
+            return
+
+        file   = view.command.exec("get_current_file")
+        if not file or not file.get_location(): return
+        buffer = view.get_buffer()
+
+        markdown_preview.popup()
+        markdown_preview.fpath = file.get_location().get_path()
+        markdown_preview._do_markdown_translate(buffer)