Fix dropdown delegating and some UI problems (#34014)

The old logic is incomplete. See the comment for the improved logic.

Fix #34011

And more fixes:

1. use empty "alt" for images, otherwise the width is not right when the
image fails to load
2. remove the "dropdown icon" patch, because it has been clearly done in
"dropdown.js" now
3. remove the "dropdown filtered item" patch, added a clear callback,
and improve the logic
4. fix global init when a node is removed and added back gain (eg: the
"cherry pick" dialog with a dropdown)

web_src/js/modules/observer.ts

@@ -46,9 +46,11 @@ function callGlobalInitFunc(el: HTMLElement) {
   const func = globalInitFuncs[initFunc];
   if (!func) throw new Error(`Global init function "${initFunc}" not found`);
 
+  // when an element node is removed and added again, it should not be re-initialized again.
   type GiteaGlobalInitElement = Partial<HTMLElement> & {_giteaGlobalInited: boolean};
-  if ((el as GiteaGlobalInitElement)._giteaGlobalInited) throw new Error(`Global init function "${initFunc}" already executed`);
+  if ((el as GiteaGlobalInitElement)._giteaGlobalInited) return;
   (el as GiteaGlobalInitElement)._giteaGlobalInited = true;
+
   func(el);
 }