[4.x] Fix ButtonGroup from overflowing

[PatrickJunod]

Change:

This PR introduce a ResizeObserver for the ButtonGroup to handle the Overflowing cases.

Fix: #9044
Fix: #9830 (Indirectly by removing the width: 1% of the button-group.css file - which was interfering with this)

Some observations:

• I did not test the performance of such a change as I'm not sure what to look for ? If someone willing to do it and knowing how could have a look ?
• I was not sure about the use of the button-group.css file. If needed, I can naturally put all css in the resources/css/elements/buttons.css file.

Example:

Example 1

When the button group can correctly be displayed as a flex row element, then the button still works the same:

Example 2

When the button group does not have enough place to be displayed correctly without overflowing, then it will be display as a vertical button group:

Example 3

This also works in other field types, like the replicator, grid, etc...:
Without overflowing:

With overflowing:

Or using the #9044 as an example:
Without overflowing:

With overflowing:

[robdekort]

Just wanted to say thank you for this PR. And what a great number huh?