Misc UI tweaks
- Use HdyHeaderBar and HdyWindow when possible
- Some small tweaks like removing yalings with floating numbers (0.802919321321)
- Replaced some GtkPopupMenus which were not menus with GtkPopup
- GtkPopupMenu is gone in gtk4 replaced one with GtkMenu
ROUNDED CORNERS
This suggests that HdyWindow should be used for the main window too.