Create actions per-window instead of per-application

Note that this changes the libkis scripting api. The Extension
class now has two methods: setup and createActions. Old code
was like this:

from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from krita import *

def hello():
    QMessageBox.information(QWidget(), "Test", "Hello World")

class HelloExtension(Extension):

  def __init__(self, parent):
      super().__init__(parent)

  def setup(self):
      action = Krita.createAction("Hello")
      action.triggered.connect(hello)

Krita.instance().addExtension(HelloExtension(Krita.instance()))

New code is like this:

from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from krita import *

def hello():
    QMessageBox.information(QWidget(), "Test", "Hello World")

class HelloExtension(Extension):

  def __init__(self, parent):
      super().__init__(parent)

  def setup(self):
      pass

  def createActions(self, window):
      action = window.createAction("Hello")
      action.triggered.connect(hello)

Krita.instance().addExtension(HelloExtension(Krita.instance()))

This also adds a new parameter to createAction: the menu location. This
is a path separated by /, for instance tools/scripts. Note that this
path must exist, otherwise a crash will happen. The paths are defined in
krita4.xmlgui...

BUG:391705

Note: we're still leaking Action objects created in Window::createAction;
that's the next fix.

CCMAIL:kimageshop@kde.org
(cherry picked from commit e9b06616)