Keep references to callbacks in Bootstrap

[kraftner]

It is great that the plugin is modular so in theory you could replace bits and pieces. But not keeping a reference to the objects hooked in Bootstrap.php makes it hard to actually do it without resorting to black magic.

[kraftner]

Considering the size of the plugin using DI is probably overkill, but a small Registry might be the way to go. Some hints on topic by @Giuseppe-Mazzapica