qml/qeapp.py: (trivial) use os.path.join for constructing path

There is an issue with plyer on Windows, which I thought was related to the mixed
backwards/forward slashes in the path, but no, this change still did not fix it:
```
  1.84 | D | gui.qml.qeapp.QEAppController | sending push notification to OS: message='heyheyhey'
Traceback (most recent call last):
  File "...\electrum\electrum\util.py", line 1111, in run_with_except_hook
    run_original(*args2, **kwargs2)
  File "...\Python310\lib\threading.py", line 953, in run
    self._target(*self._args, **self._kwargs)
  File "...\plyer\platforms\win\libs\balloontip.py", line 206, in balloon_tip
    WindowsBalloonTip(**kwargs)
  File "...\plyer\platforms\win\libs\balloontip.py", line 130, in __init__
    raise Exception('Could not load icon {}'.format(app_icon))
Exception: Could not load icon ...\electrum\electrum\gui\icons\electrum.png
```

electrum/gui/qml/qeapp.py

@@ -154,8 +154,9 @@ class QEAppController(BaseCrashReporter, QObject):
             global notification
             if not notification:
                 from plyer import notification
-            icon = (os.path.dirname(os.path.realpath(__file__))
-                    + '/../icons/electrum.png')
+            icon = os.path.join(
+                os.path.dirname(os.path.dirname(os.path.realpath(__file__))), "icons", "electrum.png",
+            )
             notification.notify('Electrum', message, app_icon=icon, app_name='Electrum')
         except ImportError:
             self.logger.warning('Notification: needs plyer; `sudo python3 -m pip install plyer`')