split $tabbar-height into $tabbar-height and $tabbar-icon-width

by splitting tab bar height in height and icon-width (respectively for height and width settings related to the tab bar and tab bar sections), you can have wider icons/clickable areas in the tab bar, controllable via _settings.
$tabbar-icon-width defaults to $tabbar-height for a square icon

Signed-off-by: Maarten Ottens <m@maartenottens.com>