Adds Eedt_panel_model methods to allow page load CSS & JS

Functionality has been added to the Eedt_panel_model::add_css and
Eedt_panel_model::add_js methods to allow specifying of multiple
assets to be loaded on page load instead of toolbar button click.

This can be accomplished by an optional second argument:

$panel = new Eedt_panel_model();
$panel->add_css("url/to/file.css", TRUE);
$panel->add_js("url/to/file.js", TRUE);