side-panel-content hide overlay button when already in fullscreen

[ljanner]

When si-side-panel-content has displayMode=overlay the overlay button does not disappear on small screen sizes where the side panel already covers the entire screen width.

Small screen, should be hidden:

Not full screen due to medium screen, is okay:

[timowolf]

@ljanner we should remove the button on the breakpoint when normal width is equal full width.

[dauriamarco]

It was regression, fixed in #1547 👍