core: switch to custom incubation controller

This change requires more QtPrivate usage but eliminates generation or
cleanup related window incubation controller bugs. Additionally it
enables async loads prior to rendering windows.

changelog/next.md

@@ -36,6 +36,8 @@ set shell id.
 - Fixed hyprland active toplevel not resetting after window closes.
 - Fixed hyprland ipc window names and titles being reversed.
 - Fixed missing signals for system tray item title and description updates.
+- Fixed asynchronous loaders not working after reload.
+- Fixed asynchronous loaders not working before window creation.
 
 ## Packaging Changes