Fix and improve registration of nodes

[MoritzBrueckner]

Fixes #1961.

Previously, the array subpackage for the array category made it impossible to use the python built-in package with the same name. It was a bit tricky to fix it but now the packages are imported correctly.

In addition to that, logic nodes are no longer registered by just importing and executing their modules, but with dedicated on_register and on_unregister functions that can be overridden if required. This makes it even simpler to register nodes and improves the maintainability.

Previously:

class MyNode(ArmLogicTreeNode).
    ...

add_node(MyNode, category="MyCategory")

Now:

class MyNode(ArmLogicTreeNode):
    arm_category = "MyCategory"

Or:

class MyNode(ArmLogicTreeNode):
    arm_category = "MyCategory"

    @classmethod
    def on_register(cls):
        do_something()
        super().on_register()

    @classmethod
    def on_unregister(cls):
        do_something_else()
        super().on_unregister()