Switched from self::url() to plugins_url() for enqueuing

[rogercoathup]

self::url() generates incorrect URL paths for enqueuing styles and
scripts on certain setups. For example: on Vagrant setups using
Chassis (https://github.com/Chassis/Chassis).

Using plugins_url() generates the correct path.

[jtsternberg]

There's a lot of history here with CMB2, but the short answer: CMB2/CMB2#27 (comment)

To address for most cases, I'll likely need to swipe from what we're doing in CMB2.

[jtsternberg]

Also, your PR complete ignores the url() method, and the filter that gets run on the URL: https://github.com/21applications/Shortcode_Button/blob/7bffb90cc1182666cbafe821ea9660c757b699f4/lib/class-shortcode-button.php#L100

[rogercoathup]

Yep, I initially tried to use WDS Shortcodes as a library (via Composer, using the same setup I use successfully for cmb2 and autoloading), but ran into problems with the TGM integration throwing errors. I don't know if that's my limited understanding of Composer configuration, or a more fundamental issue.