When embedding, `canvas.manager` may be None, but we may still have
registered the default key_press_handler (see e.g. the
embedding_in_tk_sgskip.py example), so we still need to disable the
keymap rcParams.

In order to avoid duplicating the logic as to whether use
toolmanager-cleanup in two places (and avoid things going out of sync
between begin_typing and stop_typing), register the cleanup actions
in begin_typing.

Example:
```
from matplotlib.backend_bases import key_press_handler
from matplotlib.backends.backend_qt5agg import FigureCanvas, NavigationToolbar2QT
from matplotlib.backends.qt_compat import QtCore, QtWidgets
from matplotlib.figure import Figure
from matplotlib.widgets import TextBox

class ApplicationWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()
        canvas = FigureCanvas(Figure(figsize=(5, 3)))
        canvas.setFocusPolicy(QtCore.Qt.StrongFocus)
        self.setCentralWidget(canvas)
        tb = NavigationToolbar2QT(canvas, self)
        self.addToolBar(tb)
        canvas.mpl_connect(
            "key_press_event", lambda event: key_press_handler(event, canvas, tb))
        axbox = canvas.figure.add_axes([0.1, 0.05, 0.8, 0.075])
        self._tb = TextBox(axbox, 'label')

qapp = QtWidgets.QApplication([])
app = ApplicationWindow()
app.show()
qapp.exec_()
```