[UI Overhaul] Scrollable Menus.. just the implementation for now, not actually used.

This worked initially(early testing yesterday), but since then it seems not to, either way, implementation part is out of the way, so committing.
[UI Overhaul] LLMenuGL sync
[UI Overhaul] Introduce LLMenuGL::insert from upstream and remove append/addChild wrappers in favor of insert
Introduce APP_SITE for easy rebranding of viewer website from strings.xml.