Remove support for 'Class:method' in favour of ['Class', 'method']

[anlutro]

Stays more consistent with PHP's syntax - [$obj, method] can be passed to call_user_func for example, whereas Class:method is purely made up by me.

In modern PHP versions it's also easier and looks cleaner to do [Class::class, 'method'] than Class::class.':method'.

• Add support everywhere for ['Class', 'method'] in 0.6.
• Look into deprecating 'Class:method' in 0.7.

Places to add:

• Route callables, filters, responders
• Container invoke and define factory
• Event listeners

Places to remove 'Class:method' syntax:

• Route callables, filters, responders
• Container invoke and define factory
• Event listeners