Support adding comments to decompiled function header

### Description

After https://github.com/angr/angr/pull/3909 lands in angr, the backend will support the ability to add comments to the header of the decompiled function. We still need a way to do that in the UI, though.

## Tasks
- [ ] Make the function header right-clickable and add the option to `Add comment`
- [ ] Add the hotkey `/` when clicking on the function header

All that needs to be done is adding a comment to the project `kb.comments` for the exact start address of the function.

### Alternatives

_No response_

### Additional context

_No response_