Add `siteorigin_panels_admin_row_colors` filter

[AlexGStapleton]

Resolve #968

This PR adds the siteorigin_panels_admin_row_colors filter which allows for row colour re-ordering, custom row colours with names, and row colour removal.

Add custom colour:

add_filter( 'siteorigin_panels_admin_row_colors', function( $row_colors ) {
	$row_colors['custom'] = array(
		'inactive' => '#0f0',
		'active' => '#000',
		'cell_divider' => '#ff0',
		'cell_divider_hover' => '#0ff',
	);
	
	return $row_colors;
} );

Remove lavender row colour:

add_filter( 'siteorigin_panels_admin_row_colors', function( $row_colors ) {
	unset( $row_colors[3] );
	
	return $row_colors;
} );

Reverse row colour ordering (won't change the default row color):

add_filter( 'siteorigin_panels_admin_row_colors', function( $row_colors ) {
	$row_colors = array_reverse( $row_colors, true );
	
	return $row_colors;
} );