Add headerTemplate property to Panel

[mertsincan]

Header template of the panel to customize more.

template: (options) => {
    // options.className: Style class of the default header element.
    // options.titleClassName: Style class of the title element.
    // options.iconsClassName: Style class of the icons wrapper element.
    // options.togglerClassName: Style class of the toggler element.
    // options.togglerIconClassName: Style class of the toggler icon element.
    // options.onTogglerClick: Click event for the toggler element.
    // options.titleElement: Default title element created by the component.
    // options.iconsElement: Default icons wrapper element created by the component.
    // options.togglerElement: Default toggler element created by the component.
    // options.element: Default element created by the component.
    // options.props: component props.
    // options.collapsed: Whether the panel is collapsed.
}

<Panel headerTemplate={template} .../>