Stack snackbar messages

[juancho0202]

Currently, we are missing out on bits of information if several snackbar messages pop up at the same time. As a user of OpenSCD I would like to see on the screen all snackbar messages that come at the same time stacked on top of each other.

Acceptance Criteria: If you have several snackbar messages popping up at the same time, they don't overlap but they are stacked on top of each other.

[Sander3003]

@danyill opinion: remove the snackbar messages all together; We can do this after we implemented the new notification system.

[juancho0202]

@Sander3003 @danyill stacking the messages is a smaller fix than removing the bar altogether, we only need to add the stacked attribute https://www.npmjs.com/package/@material/mwc-snackbar#variants