less: style panel headings with variables the same way as the (inverse) navbars are styled

No visible change by default, but splitting these variables out separately and
explicitly makes the system more flexible and prepare for future changes or
customizations.

@panel-primary-heading-bg already indirectly defaulted to
@kallithea-theme-main-color.

@panel-primary-text defaulted to white and would have to be customized anyway
if using a light background.