spinner: don't restart the spinner when .start() is called

Previously, when we called spinner.start(), we would automatically call
.stop() first to reset it.

Unfortunately, after calling stop(), the spinner mutates its visual
representation to become invisible using .set_text("").

This is a problem because after disappearing, the spinner does not
reappear instantly, which causes visual glitches depending on how often
we redraw the screen.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquitycore/ui/spinner.py

@@ -75,6 +75,8 @@ class Spinner(Text):
             await asyncio.sleep(self.rate)
 
     def start(self):
+        if self._spin_task is not None:
+            return
         if self.debug:
             log.debug("starting spinner %s", id(self))
         self.stop()