Replace "No note or rest selected" error dialogs with friendlier and less disruptive messaging UI

[cbjeukendrup]

Your idea

In several circumstances, we show error messages like "No note or rest selected". While they provide useful info/feedback, they are a bit disruptive.
We could solve this by presenting the same information in a more friendlier way. For example, Visual Studio Code presents not-immediately-blocking error messages in a popup in the bottom right corner:

I have been told that this design pattern is called a "toast".

We could make the message automatically disappear after 5 seconds unless the user moves their mouse over it, for example.

Additional considerations

• there are cases where we should just fix the behaviour, besides fixing the message style. For example, it should almost never matter whether the user has created a "list selection" or "range selection".
• there are cases where there is no feedback at all, for example for frames. In these cases, we should probably also show the same "toast" message.

Problem to be solved

The current dialog-style error messages scare the user and are a bit too disruptive.

Prior art

No response

Additional context

No response

Checklist

• This request follows the guidelines for reporting issues
• I have verified that this feature request has not been logged before, by searching the issue tracker for similar requests

[Fourniture]

Call me a complainer but. . . I don't like "toasts".
I actually prefer the ordinary error messages because they are standard, and because they feel more professional. To me, a toast feels tacked on. I suggest instead a different version, more similar to what is found on phones. If an error message needs to appear on a phone, it appears as a tiny translucent black capsule shape with white text. Please pardon the lack of images--the notices disappear too fast to get a picture of them. The good thing about this design is that the translucency suggests it is a temporary message, the black makes it visually distinct from the rest of the screen (which is often white), and the capsule shape (box with very rounded corners) is very non-threatening.