Replace the accordion title bar image with the caret icon of the icon theme

The accordion title bars of the Panel Wizard (Administration > Look & Feel > Panels), and of the Rights Help and Rights Tips panels, show a hard-coded icons/xwiki/accordion.png image: a grey up and down arrow.

• It doesn't follow the icon theme.
• Its color is fixed, so it doesn't adapt to the panel header color of the color theme.
• It doesn't show whether a category is open or closed.

The administration menu already solves this: its category headings render the caret-down icon of the icon theme and rotate it when the category is collapsed. The accordion title bars could do the same, using the accordion_toggle_active class that accordion.js sets on the open title bar.

Once done, the copy of the image embedded in flamingo/css/accordion.css by XWIKI-25407 can be removed.