Color/hint placeholders when editing an entry

Summary

GUI improvements on the placeholders

1. Add colours to recognized placeholders, allowing to ensure we have written a correct placeholder and help ) reading the full sequence.
2. Hint the user when he starts typing { with something like a dropdown suggesting frequent placeholders that match what he has already typed.
3. Hint when a valid reference placeholder links to nothing or to an empty field.
4. For the REF placeholder, a helper to select another entry to avoid the necessity to copy the target's uuid manually, and show what valid REF placeholders link to using a tooltip or by making them clickable.
5. Provide a small button at the end of any text area containing placeholders to previsualize the result.
6. To the drop down that suggests frequently used usernames, add buttons to link to the selected username instead of simply copying it.

Examples

Some example in context with random colors

And the button to generate reference instead of copying usernames:

Context

I think placeholders are a really powerful feature, especially the {REF:...} that allow linking entries that depeend on the same thing. But I often found them hard to write and hard to read when the sequence becomes longer.

Note

I know Qt, so if you want I can work on a PR for that.