Support multiple boot using callbacks

[ralphjsmit]

Description

I generally use the bootUsing() function quite often to provide a callback. One technique is use is to have an abstract base PanelProvider class and the actual panel provider extend this abstract class. That way, panel configuration can be applied to multiple panels at once. Currently, if I provide a bootUsing() in both the abstract base class and the actual panel provider that extends this, the bootUsing() callback provided first will be overwritten by later calls. I remember seing a PR about this earlier, but I couldn't find it, so it could've imagined it. Another risk relating to this is when registering plugins, they also get access to the $panel object and could (at any time) provide a custom bootUsing(), which could then overwrite the developer's bootUsing() from the panel provider class. I therefore thing it would be useful to store the callbacks as an array and call them one by one, in order to prevent such over-writing issues (and also make it safe for plugins to use bootUsing()).

Thanks!

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.