`ginput` works even for figures that are not created by `plt.figure()`
(and friends -- typically, that means we're embedding matplotlib in a
GUI app)... except that `ginput` tries to call `fig.show()` to raise the
figure, which would fail.

Instead, only call `fig.show` if the figure has a manager (i.e., was
created by `plt.figure`).

Example (click once to start point selection, then make further clicks
as in a normal `ginput` call):

```
import matplotlib; matplotlib.use("qt5agg")
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg
from matplotlib.figure import Figure
from PyQt5.QtCore import Qt
from PyQt5.QtWidgets import *

class Win(QMainWindow):
    def __init__(self):
        super().__init__()
        self.fig = fig = Figure()
        canvas = FigureCanvasQTAgg(fig)
        self.setCentralWidget(canvas)
        fig.add_subplot(111)
        self._cid = fig.canvas.mpl_connect(
            "button_press_event", self._start_ginput)

    def _start_ginput(self, event):
        self.fig.canvas.mpl_disconnect(self._cid)
        # Workaround for now; patch makes this unnecessary.
        self.fig.show = lambda: None
        print(self.fig.ginput(-1, timeout=3))

app = QApplication([])
win = Win()
win.show()
app.exec_()
```