A `Plugin` object contains a collection of nodes, represented as
`NodePlugin` objects. This commit adds the implementation of the `Plugin`
class with the methods to add and remove `NodePlugins`, as well as an
empty class for the `NodePlugin` objects themselves.

The validation of the node descriptions will be handled directly within
the `NodePlugin` objects and there is thus no need for this method to
exist outside of plugins.py.

The `NodePluginManager` class manages is used to load, manage,
and register plugins and nodes.

Templates that are available for a plugin are detected and gathered
upon the plugin's creation.

Nodes are now registered and unregistered through the `NodePluginManager`.

Also comply more with PEP8 linting rules.

The plugin manager is now effectively used for all the operations that
involve registering or unregistering nodes.

…...`

This prevents ambiguities between `NodePlugin` objects that have been
registered (and are thus instantiable) and those that belong to `Plugin`
objects but have not been registered.

A `NodePlugin` may be part of a `Plugin` even if it has not been
registered. `containsNode` allows to check whether a `NodePlugin` is
contained within a `Plugin`, and `belongsToPlugin`

This will replace all the nodes in the current graph with a new instance
of the same type. If any change has occured in the description of these
nodes since their last instantiation, the new nodes will reflect them.

"Reload All Nodes" will reload all the descriptions for `NodePlugins`
that are part of loaded plugins, whether they are registered or not.

Once the reload of the descriptions has been performed, the node instances
from the current graph will be reloaded as well to reflect any change
in the description.

…atus

If the plugin has successfully been reloaded, return `True`. If it has
not been reloaded for any reason (either an error or because no
modification has been made to it since it has been loaded last), then
return `False`.

When requesting a `NodePlugin` to reload, the status of the reloading
is now saved. If it has been successful, all the nodes of this type in
the graph will effectively be removed. If it has not (either because of
an error, or because no change has been done to the description), then
nothing will happen.

Replacing only the nodes whose description has been updated prevents
from having to update heavy graphs for very few nodes to change (if any).

… exist

Warning only on a module/file without any element in it and provide
full-path to ease debugging.
For package/folder, it does not generate any message.