Added explicit height to Metabox titles

If somebody puts a br tag in a metabox title, it throws off the whole layout - so this forces the title to max out at 15 pixels with the overlay being hidden
Swap custom wp_kses array for wp_kses_allowed_html

WordPress has a built in array for wp_kses for posts - using that instead of custom array. I didn't know this existing until today.
Recude number of checks needed for Theme License

We were checking way too much
Plugin API won't call twice if it's already called

This saved resources by limiting the number of calls the api makes upon updating etc. It uses a class-wide variable to check if we've already checked the api for plugin updates