For linked or overriden data-blocks, show a menu with operations:
- Make Local (regular make local or remove the override and make local)
- Add Library Override
These options were available before as a icon to click on. But to create a library override there was a hack so you could Shift-click it, which is wasn't meant as final solution in the first place.
A menu makes things more discoverable and scales better (more operations can be added).
- Use the red "disabled hint" in tooltips to explain why buttons are disabled, rather than using the regular tooltip text for that. That is the consistent way to do that now.
- Show library link status icon in the ID-template text button. Useful to indicate that something is a linked data-block.
(Should probably read "cannot be changed", I just kept the old text.)